[Updated at 5:03 p.m.] Police officers shot the suspect at about 4:50 p.m., Montgomery County Police Chief J. Thomas Manger told reporters. All three hostages are safe and out of the building.
"The suspect is in custody," he said. Manger said he had no information about the suspect's condition.
A number of devices in backpacks have to be rendered safe, he added.
During negotiations, "he had a wide range of emotions," he said.
[Updated at 4:14 p.m.] Montgomery County Police spokesman Paul Starks told reporters there remained no reports of injuries, but he could not confirm whether everyone had been evacuated from the building, nor would he say how many hostages were being held.
"We're just continuing to talk to the man and we're hoping to bring about a safe and successful resolution," Montgomery County Police spokesman Paul Starks told reporters.
"It's my understanding that Montgomery County police officers are speaking with him," he said, adding that he believed the conversation was being held by phone. "That has been continuing with him for well over an hour, closing in on two hours now."
[Updated at 4:04 p.m.] Aaron Morrissey, the editor in chief of the web publication DCist, said he came across James Lee's anti-Discovery Channel manifesto in 2008, when Lee was planning to hold a protest against the channel.
Lee has been identified by law enforcement sources as the man holding hostages at the Discovery Channel headquarters in Silver Spring, Maryland.
Lee, Morrissey said, "had some kind of off-the-wall ideas in that manifesto."
The 2008 protest, he said, "was not that well attended."
A month or so later, Lee was arrested near the building on littering and disorderly conduct charges, Morrissey said. The littering stemmed from Lee's throwing money in to the air, he said.
[Updated at 3:41 p.m.] A man with the same name and age as the suspect in a hostage situation at the Discovery Channel headquarters in Silver Spring, Maryland, was found guilty in 2008 of disorderly conduct, according to court documents.
James Lee, 43, was acquitted of littering in the same case, according to Montgomery County, Maryland, Circuit Court records, according to Eric Nee, a senior assistant state's attorney.
Lee's two-year supervised probation ended August 18, records show.
[Updated at 3:31 p.m.] Montgomery County, Maryland, Police Chief J. Thomas Mangersaid the gunman at the Discovery Channel's headquarters is holding hostage more than one person.
Manger said about 3:20 p.m. that negotiations have been going on for nearly an hour, "and we will continue that for as long as we can."
[Updated at 3:17 p.m.] The man suspected of taking at least one hostage at the Discovery Channel's headquarters entered the building's main entrance "wearing what appeared to be metallic canister devices on his front and back," a police spokesman said Wednesday. "He also pulled a handgun out and was waving a handgun."
Silver Spring Police Chief. Chief J. Thomas Manger said police haven't confirmed whether any shots were fired inside the building.
Manger said Lee has remained in first floor area of the Discovery building, where 1,900 employees work.
"We believe that most of them are out," Manger said. "There are a small number of hostages with him."
[Updated at 3:10 p.m.]
James Lee, the man suspected of taking a hostage at the Discovery Channel headquarters, is an environmental protester who has been publishing criticisms of the network, according to a senior law enforcement official close to the investigation.
An angry manifesto posted on a website called SaveThePlanetProtest.com repeatedly refers to humans as "filth" and demands that the Discovery Channel
"stop encouraging the birth of any more parasitic human infants."
"Civilization must be exposed for the filth it is," the 1,149-word statement says.
"Humans are the most destructive, filthy, pollutive creatures around and are wrecking what's left of the planet with their false morals and breeding culture," it continues.
Lee also blasts immigration, farming, weapons of mass destruction, automotive pollution, "and the whole blasted human economy."
He demands that the Discovery Channel broadcast daily prime-time shows devoted to "solutions to save the planet," perhaps in a game-show format, insisting "Make it interesting so people watch and apply solutions!!!!"
Many of Lee's comments are directed at "the media," saying "You can reach enough people. It's your responsibility because you reach so many minds!!!"
"The world needs TV shows that DEVELOP solutions to the problems that humans are causing, not stupify the people into destroying the world. Not
encouraging them to breed more environmentally harmful humans," the manifesto says.
"These are the demands and sayings of Lee," the manifesto concludes.
[Updated at 2:51 p.m.] The man believed to be holding at least one hostage at the Discovery Channel headquarters in Silver Spring, Maryland, has been identified as James Lee, a law enforcement source tells CNN.
"We can confirm that he is armed and he does have some sort of package with him," said Scott Graham of the Montgomery County, Maryland, Fire Department. Asked if that means Lee has as bomb, Graham said he could not say.
Police remain in contact with him, Graham said.
He said there have been unconfirmed reports of a shot fired, but there have been no reports of injuries. "It would be premature to speculate on any exact causes as to why this is occurring," he added.
He added that some people may remain in the building.
[Updated at 2:46 p.m.] Capt. Paul Starks said that police were in communication with the gunman, and that he was making some demands. The suspect had "concerns" with the Discovery Corporation, he said.
[Updated at 2:37 p.m.] CNN affiliate WJLA is reporting there was an ambulance on scene and a stretcher was seen being taken into the lobby.
[Updated at 2:34 p.m.] Lorie Rorae, an evacuated employee who worked on second floor, said security said to leave the building.
Rorae said the security officer said two colleagues were being held hostage.
Rorae said there was an announcement telling employees what to do and where to go.
"It was orderly," Rorae said.
[Updated at 2:31 p.m.] Montgomery County Police said they are speaking with a man who has taken a hostage. Police described the man's grudge the following way: "His beef is with Discovery."
"We are speaking with him now," police said.
[Updated at 2:31 p.m.] A witness, Lisa Lucas, told CNN affiliate WJLA that she and others outside of the building heard at least one gunshot from One Discovery Place. Friz said that to the best of his knowledge no shots had been fired by police.
[Updated at 2:19 p.m.] The Discovery Channel day care has been evacuated to a nearby area, according to CNN affiliate WJLA. The day care had about 100 kids, WJLA reported.
[Updated at 2:18 p.m.] The FBI's bomb techicians and ATF agents have been sent to the scene of the hostage situation in a Washington suburb, spokesmen from both agencies said.
[Updated at 2:17 p.m.] Renard Jenkins, who works across from the Discovery Channel headquarters where the hostage drama is unfolding, said he sees the mobilization of a bomb squad and a large armored vehicle pulling a trailer.
"Police are on high alert, positioning themselves behind columns, cars, anything that gives cover. All have their weapons drawn," Jenkins said. "From what we're seeing, everyone seems to be calm."
He said he cannot see into the lobby where the hostage-taking is apparently under way, but can see the activity immediately outside.
[Updated at 2:07 p.m.] CNN affiliate WJLA is reporting that police told them a man entered the lobby and may have fired a weapon and declared, "Nobody is going anywhere."
[Updated at 1:52 p.m.] A SWAT team and additional resources are responding, said Montgomery County police Cpl. Dan Friz. Authorities were trying to get in touch with the man believed to have taken a hostage.
It appears the man has some kind of explosive device on his person, Friz said. Fire marshals with explosives experience were assisting.
[Updated at 1:44 p.m.] The Discovery Channel sent an e-mail to all employees in the building at 1:21 p.m. telling them they "have reason to believe there is an armed gunman at One Discovery Place."
[Updated at 1:39 p.m.] At least one hostage is confirmed at the headquarters of the Discovery Channel in Silver Spring, Maryland, police said.
[Posted at 1:34 p.m.] A police emergency response team was responding to reports of a man with a gun and possible explosives at the headquarters of the Discovery Channel in Silver Spring, Maryland, a police spokeswoman said.
Montgomery County police received the call at about 1 p.m., said Angela Cruz. The call came from One Discovery Place, the channel's headquarters.
The area has been evacuated, Cruz said. No further information was immediately available.