[Update, 11:05 a.m. ET] This post is no longer being updated. For Tuesday's coverage, please read this story.
[Update, 6:46 a.m. ET] Overnight, President Barack Obama received updates from his assistant for homeland security and counterterrorism Lisa Monaco on the ongoing response efforts and investigation into the explosions in Boston, including the continuing federal support for those activities, a White House official said, according to CNN's Lesa Jansen. The president made clear that he expects to be kept up to date on any developments and directed his team to make sure that all federal resources that can support these efforts, including the investigation being led by the FBI, be made available, the official said.
[Update, 4:32 a.m. ET] President Hamid Karzai expressed grief over the civilian casualties caused by two bomb explosions near the Boston Marathon on Monday.
Denouncing the terrorist attacks in the strongest possible terms, President Karzai said, “Having suffered from terrorist attacks and civilian casualties for years, our people feel better the pain and suffering arising from such incidents.”
President Karzai offers condolences and sympathies to the families of the victims and the people of the United States of America.
[Update, 4:27 a.m. ET] A huge wave of strangers is greeting the many visitors stranded by the Boston Marathon bombings with a massive outpouring of support. "We figure this is the least we can do," said Heather Carey, who offered a couch at the home near Boston University she shares with roommates. "I saw a website with many others offering their spaces like we did. It is awesome to see so many people helping."
The twin blasts Monday that left three dead and more than 140 wounded also left countless people without shelter. Investigators turned the heart of Boston into a crime scene, evacuating several hotels. This left dozens of visitors - some of them international runners unfamiliar with the area - stranded.
By Monday evening, pleas were posted on several websites.
"Me and my friends lost our phone after the explosion," a woman posted on Reddit. "We are visiting from Korea so our English be not very good. My friend is in the hospital now and they say we can not stay over night in hospital."
Another woman posted: "I have nowhere to go."
Quickly, the online cries for help were answered. Websites were flooded with Bostonians offering aid. Even though it was unclear how many people were helped, by early Tuesday morning a Facebook page set up for victims listed more than 100 people offering rooms and rides.
[Update, 4:09 a.m. ET] The Mayor of London Boris Johnson said: "The bombings in Boston are shocking, cowardly and horrific, and the thoughts of all Londoners this morning will be with the victims. Boston is a proud City built on history, tradition and a real sense of community. These attacks were aimed at its core, at innocent men, women and children enjoying a Spring day out at a major sporting event. We do have robust security measures in place for Sunday's London Marathon, but given events in Boston it's only prudent for the police and the organisers of Sunday's race to re-examine those security arrangements."
[Update, 3:35 a.m. ET] The identity of the child killed in Monday’s bombings at the Boston Marathon is 8-year-old Martin Richard, according to The Boston Globe. The newspaper also reported residents in Dorchester gathered at a local restaurant to remember the child Monday night.
[Update, 3:22 a.m. ET] Dr. Ron Walls of Brigham and Women's Hospital, which received 31 patients, said the debris found in some patients' wounds did not appear to be from ball bearings.
"Everything we saw was sort of ordinary ambient material that could have been propelled by the blast but was not added to the device," Walls said. "It was not the kind of things that would be added to a device to make it more injurious than it otherwise would be."
At Massachusetts General Hospital, several patients suffered from injuries to lower limbs that will require "serial operations" in the coming days, trauma surgeon Peter Fagenholz said Monday night.
He said the most serious wounds "have been combined, complex lower injuries that involve blood vessels, bone and tissue."
[Update, 2:56 a.m. ET] Chief Superintendent Julia Pendry, event commander for the London Marathon, said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
[Update, 2:15 a.m. ET Tuesday] At least 17 people are reported to still be in critical condition. The full horror of Monday's bomb attacks in Boston was reflected in emergency rooms across the city as doctors were forced to perform amputations and treat injuries normally expected on a battlefield.
[Update, 11:41 p.m. ET Monday] Stephen Segatore, a nurse who was at the medical tent near the finish line for the Boston Marathon, said emergency responders immediately went into mass-casualty mode.
"We had full trauma response at the scene," he told CNN. "We had physicians, nurses who are experienced in trauma care. We had EMTs and it was a full Level 1 trauma experience."
Segatore said he treated at least 25 people as those experienced in trauma care stepped forward while others treated people with minor injuries.
[Update, 11:35 p.m. ET] Saudi ambassador to the United States Adel Al-Jubeir condemned the bombings in Boston and offered his condolences to victims' families.
“What occurred today in Boston is a heinous crime which contradicts the values of humanity.” he said.
[Update, 10:52 p.m. ET] The total of injured has risen to 144 people, officials at Boston area hospitals said. That includes three additional patients at Brigham and Women’s Hospital.
[Update, 10:41 p.m. ET] A law enforcement source in Boston tells CNN that investigators have a "number of active leads, and some good early progress in the forensics analysis."
[Update, 10:07 p.m. ET] Dr. Peter Fagenholz told reporters that there were 29 wounded people at Massachusetts General Hospital, eight of whom were in critical condition. Many of the people had shrapnel injuries to their lower extremites, he said.
"We have performed several amputations," he said.
There were no pediatric patients among the wounded, he said.
[Update, 9:38 p.m. ET] Dr. Allan Panter, who was near the finish line waiting for his wife who was running the race, told CNN he was standing about 20 to 25 feet from the first blast. He said he treated victims on the street after the explosion.
"I saw at least six to seven people down next to me," he said. "They protected me from the blast. One lady expired. One gentleman lost both his (lower) limbs. Most of the injuries were lower extremities. I could not figure out why the young lady had expired. I could not find any injury on her thorax."
[Update, 9:28 p.m. ET] Bill Iffrig, seen in video wearing an orange tank top and being blown over as he approached the finish line, told CNN's Piers Morgan that he was feeling OK after the blast.
"I got down to within about 15 feet of the finishing apron and heard just tremendous explosion, sounded like a bomb went off right next to me, and the shock waves just hit my whole body and my legs just started jittering around," he said. "I knew i was going down and so i ended up down on the blacktop."
Iffrig, 78, said he was assisted by one of the event volunteers, who helped him up so he could finish the race. After that, the worker looked for aid for Iffrig, who had just a scratch from his fall.
"He insisted on getting a wheelchair over there so we started to do that, but then before that was rounded up, i said my hotel's about six blocks away so I think I can make it okay. So they let me get out of there and I went on home to my wife."
[Update, 8:55 p.m. ET] A Saudi national with a leg wound was under guard at a Boston hospital in connection with the bombings at the Boston Marathon, but investigators cannot say he is involved at this time and he is not in custody, a law enforcement official said Monday evening.
[Update, 8:54 p.m. ET] Three people were killed in the bombings, Boston Police Commissioner Ed Davis told reporters Monday night, raising the toll by one.
[Update, 8:52 p.m. ET] The FBI is taking the lead in the investigation, Rick DesLauriers, FBI Special Agent in Charge of the Boston field office, told reporters.
[Update, 8:44 p.m.ET ] The Boston Celtics home game against the Indiana Pacers, originally scheduled for Tuesday, was canceled, the NBA announced. With the regular season almost at its end, the contest will not be made up.
[Update, 8:36 p.m. ET] Investigators have warned law enforcement officers to be on the lookout for a "darker-skinned or black male" with a possible foreign accent in connection with Monday's bombings at the Boston Marathon, according to a law enforcement advisory obtained by CNN.
The man was seen with a black backpack and sweatshirt and was trying to get into a restricted area about five minutes before the first explosion, the lookout notice states.
[Update, 8:35 p.m.] Hospital workers have treated 141 people after the Boston Marathon bombings, officials at those facilities said Monday night. Two people died in the terror attack, including an 8-year-old boy, a state law enforcement source said.
[Update, 8:32 p.m.] A statement has been issued by the race organizers: "The Boston Athletic Association extends its deepest sympathies to all those who were affected in any way by todays events.
"Today is a sad day for the City of Boston, for the running community, and for all those who were here to enjoy the 117th running of the Boston Marathon. What was intended to be a day of joy ...and celebration quickly became a day in which running a marathon was of little importance.
"We can confirm that all of the remaining runners who were out on the course when the tragic events unfolded have been returned to a community meeting area.
"At this time, runners bags in Boston which remain unclaimed may be picked up by runners presenting their bib number or proof of race participation at the Castle, at 101 Arlington Street, in Boston.
"At this time, we are cooperating with the City of Boston, the Commonwealth of Massachusetts, and all federal law enforcement officials.
"We would like to thank the countless people from around the world who have reached out to support us today."
[Update, 7:57 p.m. ET] Doctors are "pulling ball bearings out of people in the emergency room," a terrorism expert briefed on the investigation told CNN's Deborah Feyerick.
The same source said the blasts resulted in at least 10 lost limbs.
[Update, 7:43 p.m. ET] An 8-year-old boy was among those killed, a state law enforcement source said, according to CNN's John King.
[Update, 7:38 p.m. ET] At least 132 people – including eight children – have been injured in the bombings, according to Boston-area hospitals. Boston police earlier said that two people were killed.
At least 17 of the injured are in critical condition, and at least 25 are in serious condition, area hospitals said.
[Update, 7:08 p.m. ET] A witness, Marilyn Miller, told CNN that she was about 30 feet away from the first bomb when it went off. The second bomb came about 12 seconds after and about 50 to 100 yards away from the first, according to authorities and an analysis of video from the site.
Miller was waiting for a runner who, it turns out, was probably about 10 minutes away from the finish line.
"We saw injuries all around us," Miller said. Someone was putting pressure on a woman's neck. "A little boy, his leg was torn up. A woman, (people) were (shouting), 'Critical, critical, get out of out way!'"
[Update, 6:51 p.m. ET] At least 110 people have been injured in the bombings, according to Boston-area hospitals.
[Update, 6:49 p.m. ET] Boston cell phone services were overloaded in the wake of the blast, slowing the city's network dramatically and hampering the investigation in the early going, federal law enforcement officials told CNN.
Unconfirmed rumors began circulating on social media and elsewhere that law enforcement had shut down cell service to prevent more explosives from being detonated remotely. But mobile companies were saying that was never the case, CNN's Doug Gross reports.
"Verizon Wireless has not been asked by any government agency to turn down its wireless service," a spokesman for that company told CNN. "Any reports to that effect are inaccurate."
In other media reports, Sprint similarly denied being asked to shut down service.
Online, Bostonians were being encouraged to stay off of their mobile phones except for emergencies and even open up their wireless connections to help take the load off of the cellular data network.
"If you live or run a business in #Boston near bombsite (please) open your wifi for people to use," tweeted Disaster Tech Lab, an Irish nonprofit dedicated to providing technology to assist in emergency situations.
[Update, 6:47 p.m. ET] Initial tests indicate that the two bombs were small and possibly crude, with the tests not indicating any high-grade explosive material was used, a federal law enforcement official with knowledge of the investigation told CNN national security contributor and former homeland security adviser Fran Townsend.
The source said the FBI considers the incident a terrorist attack, "but they've made clear to me they do not know at this time whether those responsible for the attack were a foreign or domestic group," Townsend said.
[Update, 6:35 p.m. ET] U.S. Rep. Bill Keating, D-Massachusetts, said an unexploded device was found at a hotel on Boylston Street, and another unexploded device was found at an undisclosed location.
Keating, who is a member of the House Homeland Security committee and has spoken to law enforcement sources, tells CNN's Dierdre Walsh that the incidents were a "sophisticated, coordinated, planned attack."
[Update, 6:14 p.m. ET] More from President Obama, who just wrapped up his brief statement at the White House: "We still do not know who did this or why ... but make no mistake: We will get to the bottom of (this). We will find out who did this. We will find out why they did this. ... Any responsible groups will feel the full weight of justice."
[Update, 6:11 p.m. ET] President Barack Obama is speaking about the bombings now: “The American people will say a prayer for Boston tonight, and Michelle and I send out deepest thoughts and prayers to the victims," Obama said at the White House.
[Update, 5:59 p.m. ET] John Manis, an eyewitness in his 50s, was about 200 feet away from the finish line near the Prudential building when the bombings occurred. He felt the blast to the point that it made him and others around him jump in the air, and some others around him fell down on the ground, he said, according to CNN's Eden Pontz.
Manis said he heard two blasts about five seconds apart. He said there was confusion all around him, and he was hustled into the nearby Mandarin Hotel. Officials wouldn’t let them leave the hotel for a bit, and he says all who were there were all frisked by police. He said that when he left, he saw broken storefronts and lots of blood.
[Update, 5:51 p.m. ET] President Barack Obama is expected to deliver a statement at about 6:10 p.m. ET from the White House.
[Update, 5:35 p.m. ET] Google has established a person-finder related to the Boston bombings. People who are looking for someone or have information about someone can make reports there.
[Update, 5:31 p.m. ET] Boston police now appear to be backing away from their commissioner's earlier statement that a third incident – at the JFK Library 5 miles from the finish line - might have been related to the Boston Marathon blasts.
On Twitter, Boston police say: "Update JFK incident appears to be fire related."
[Update, 5:21 p.m. ET] Precautions are being taken at the White House because of the Boston explosions, CNN’s Jessica Yellin reports. See that in the video below, as well as Vice President Joe Biden's reaction to the incident:
[Update, 5:17 p.m. ET] In the video below, a man describes the initial blast, saying the impact was so strong it “almost blew my head off.” He was not injured, but saw many people sustain horrific injuries.
[Update, 5:15 p.m. ET] The Boston Globe is reporting a much higher injury count. They report that more than 100 people are being treated for injuries, citing local hospitals.
[Update, 5:10 p.m. ET] Hospitals now say they are treating as many as 51 wounded after the bombings. Two people have been killed, according to Boston police.
[Update, 5:09 p.m. ET] It will take a long time to clear the area, because lots of people dropped bags and whatever else they had when the finish-line blasts happened. Authorities have to check all of those bags, and bomb squads "may be blowing things up over the next few hours" out of precaution, Boston Police Commissioner Ed Davis said.
In the words of Boston Globe political reporter Cynthia Needham, on Twitter:
Side problem, according to commissioner: People running from scene dropped bags, and personal belongings in the street. All must be checked.—
Cynthia Needham (@globecynthia) April 15, 2013
Thousands of runners still had yet to finish the race when the bombs exploded in a spectator area along Boylston Street near the finish line, CNN executive producer Matt Frucci at the scene.
[Update, 4:58 p.m. ET] New details from Boston Police Commissioner Ed Davis:
– A third explosion happened at the John F. Kennedy Presidential Library "about a half-hour ago." The library is about 5 miles southeast of the Boston Marathon finish line.
– Police don't immediately know whether that explosion is related to the two near the Boston Marathon finish line.
– The two blasts near the finish line – along Boylston Street near Copley Square - "happened 50 to 100 yards apart."
– "We're recommending to people that they stay home ... and that they don't go anyplace and congregate in large crowds."
– Relatives of people who may be missing in the area can call the mayor's hotline at 617-635-4500.
– Anyone who has information about the bombings or saw anything suspicious can call 1-800-494-TIPS.
[Update, 4:46 p.m. ET] Massachusetts Gov. Deval Patrick says “this is a horrific day in Boston."
"My thoughts and prayers are with those who have been injured," Patrick said in a statement released this afternoon. "I have been in touch with the president, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.”
[Update, 4:45 p.m. ET] It appears that so many people are using cell phones in the center of Boston, consistent service is hard to get – and the overload is hampering the investigation, two federal law enforcement sources tell CNN.
[Update, 4:40 p.m. ET] Another journalist says she saw victims who lost limbs. This account is from Boston Globe political reporter Cynthia Needham:
Outside MGH: Head of emergency medicine says 19 have been brought to MGH, six critically injured, some with amputations.—
Cynthia Needham (@globecynthia) April 15, 2013
"Outside MGH: Head of emergency medicine says 19 have been brought to MGH, six critically injured, some with amputations," she posted to Twitter.
Earlier, we noted that Boston.com sports producer Steve Silva reported that he "saw dismemberment" and "blood everywhere."
[Update, 4:37 p.m. ET] Organizers with the London Marathon, scheduled for this coming Sunday, have taken notice.
"We are deeply saddened and shocked by the news from Boston," London Marathon officials said Monday. "Our immediate thoughts are with the people there and their families. It is a very sad day for athletics and for our friends and colleagues in marathon running. Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
[Update, 4:30 p.m. ET] Boston firefighters have found what they believe is an unexploded device after the blasts, a government official said, according to CNN's Joe Johns.
[Update, 4:27 p.m. ET] "I saw blood everywhere," Boston.com sports producer Steve Silva told Boston.com.
Silva told the news outlet that he was near the finish line when the explosions happened. He said he saw a number of injuries in the area where spectators were. He saw "someone lost their leg," and he said "people are crying, people are confused."
"It was just an explosion, it came out of nowhere," he said. "There are multiple injuries. I saw dismemberment, I saw blood everywhere. People are badly injured."
[Update, 4:19 p.m. ET] We have a new injury count: According to hospital officials, at least 28 people are being treated for injuries connected to this afternoon's blasts near the Boston Marathon finish line.
Nineteen were being treated at Massachusetts General and nine at Tufts Medical Center, officials at those facilities said. Boston police earlier put the number of victims at two dead and 22 hurt.
[Update, 4:16 p.m. ET] "People started scrambling, pushing, shoving" when the explosions happened in a sidewalk area along Boylston Street, near the finish line in the Copley Square area, says CNN executive producer Matt Frucci at the scene.
Frucci said he heard the blasts.
"After the dust settled, (I saw) six or seven people strewn about the area where the second (explosion) was.
[Update, 4:11 p.m. ET] A Red Cross website has been established to help people find loved ones in the area.
"Individuals can register themselves as safe or search for loved ones," Massachusetts' emergency management agency says.
[Update, 4:08 p.m. ET] At least two people have been killed and 22 are injured in the apparent bombings near the finish line of the Boston Marathon, Boston police say.
[Update, 4:02 p.m. ET] A Massachusetts General Hospital spokeswoman tells CNN 19 victims have been brought in.
[Update,3:57 p.m. ET] On their Twitter page, Boston marathon officials made this announcement: "There were two bombs that exploded near the finish line in today's Boston Marathon. We are working with law enforcement to understand what exactly has happened."
[Update, 3:53 p.m. ET] New York is taking precautions as a result of the explosions at the Boston Marathon.
In a written statement, New York Police Department Deputy Commissioner Paul Browne said: "We're stepping up security at hotels and other prominent locations in the city through deployment of the NYPD's critical response vehicles until more about the explosion is learned.
[Update, 3:45 p.m. ET] Paramedics were treating several victims at the scene, and police ordered onlookers to back away from the area. Troops from the Massachusetts National Guard were assisting police as well.
Onlooker Josh Matthews said he heard the blast, then saw police running toward the scene.
"We just heard a lot of sirens, and people were kind of frantic, and it was a bad situation, so we got out of there," he said.
[Update, 3:37 p.m.] Four victims of explosions near the Boston Marathon finish line are at the emergency room at Massachusetts General Hospital, a hospital spokeswoman told CNN. She had no information about the victims' conditions.
[Posted at 3:25 p.m. ET] A pair of explosions rocked the finish line at the Boston Marathon on Monday afternoon, injuring at least a half-dozen people, a CNN producer at the scene said.
The blasts occurred a few seconds apart, shrouding downtown Boston's Copley Square in smoke. Paramedics were treating several victims at the scene, and police ordered onlookers to back away from the area, CNN Producer Matt Frucci reported.
The explosions occurred about 2:45 p.m., about an hour after the first runners had crossed the finish line, Frucci said.