Editor's note: The FBI and the DEA have disrupted a plot involving Iran to kill the Saudi ambassador to the United States and commit other attacks, according to the U.S. Justice Department. Two men - one arrested, the other at large - have been charged in connection with the plot, which the Justice Department says was directed by elements of the Iranian government.

The Justice Department says one of the men - Manssor Arbabsiar, a naturalized U.S. citizen holding an Iranian passport - arranged to hire for the assassination someone in Mexico who he thought was an associate of a drug trafficking cartel. The person in Mexico actually was a DEA confidential source who was posing as a cartel associate, the Justice Department says.

Follow below for the latest developments and read the Justice Department complaint (PDF).

[Updated at 8:09 p.m. ET]  In their investigation into an alleged plot to kill Saudi Arabia's ambassador to the United States, federal agents interrogated suspect Manssor Arbabsiar for 12 days, a senior counterterrorism official said Tuesday.

Cooperation from Mexican officials played a key role in the investigation, the official said. U.S. authorities arranged with Mexican officials for Arbabsiar to be denied entry into Mexico, the official said.
From there, he was placed on an airplane to New York, where he was taken into custody and quietly taken to a U.S. government facility, the counterterrorism official said. U.S. authorities interviewed him there every day and compiled dozens of intelligence reports.

[Updated at 8:04 p.m. ET] Georgia Sen. Saxby Chambliss released a statement Tuesday, congratulating the FBI, DEA and other federal agencies involved in the case.

"While I believe our justice system will deal appropriately with the defendant in custody, our government must also deal with the Iranian regime," Chambliss said. "In addition to allegedly sponsoring this plot, Iran has supported and provided weapons for attacks on our soldiers in Iraq and Afghanistan. This has continued far too long with no repercussions."

[Updated at 7:41 p.m. ET] Iran's UN Ambassador Mohammad Khazaee responded to U.S. accusations that the Iranian government was involved in a murder-for-hire plot Tuesday night, telling CNN's Erin Burnett that the whole thing was "a lie."

“The whole issue is a provocation against Iran," Khazaee said. "We strongly reject this accusation," he said, suggesting that the narrative was a "blatant" ploy by Washington to terrorize the American public.

[Updated at 7:41 p.m. ET] Tom Kean, former chairman of the 9/11 Commission said the alleged plot "surprises me." Speaking to CNN's Erin Burnett, Kean said the plot is "pretty close to an act of war. You don’t go in somebody’s capital to blow somebody up.”

[Updated at 7:07 p.m. ET] New York Rep. Peter King, speaking to CNN’s Erin Burnett, said the alleged Iranian plot should be taken seriously by U.S. officials. “This would have been an act of war [if carried out]. It has raised this relationship, between the United States and Iran, to a very precipitous level,” King said.

“This violates all international norms, it violates all international laws. ... We can’t allow this to go without a strong reaction," King, who chairs the Homeland Security Committee," told CNN.

After saying he would back whatever action the administration might take, King said “we should at least consider a sign of military action.  ... something to indicate how seriously we're taking this." He added that U.S. officials should even consider removing Iranian diplomats from the country. “I think everything should be kept on the table.”

[Updated at 6:52 p.m. ET] A spokesman for Iranian President Mahmoud Ahmadinejad said Tuesday that the alleged plot was "a child's story" and "a fabrication."

The Iranian government was awaiting details about the accusations, spokesman Ali Akbar Javanfekr said. He suggested U.S. authorities were attempting to distract American citizens.

"They want to take the public's mind off the serious domestic problems they're facing these days and scare them with fabricated problems outside the country," he said.

[Updated at 6:40 p.m. ET] Mexican immigration officials blocked Manssor Arbabsiar, now accused of plotting to kill Saudi Arabia's ambassador to the United States, from entering Mexico last month, Mexico's foreign ministry said in a statement released Tuesday.

Mexican immigration authorities blocked his entry because of an arrest warrant issued by the United States, the foreign ministry said. U.S. authorities later arrested him in New York, it said.

[Updated at 5:57 p.m. ET] U.S. National Security Adviser Tom Donilon in recent weeks went to Saudi Arabia to brief Saudi King Abdullah on the terror plot, a senior administration official familiar with the terror plot said, according to CNN Chief Political Analyst Gloria Borger.

The Saudis were “outraged" not only because of the plot, but because the ambassador is "someone who is close to the king,” the official said.

The Obama administration has specific information tying senior officials in Iran's Quds Force - a special unit of the Iranian Islamic Revolutionary Guard Corps - the official said. One question, according to the official, is whether the Quds officials were freelancing or got approval from senior officials in the Iranian regime.

The administration intends to “go to other countries and say this is a serious escalation of Iran’s use of political violence,” the official said. “Some may build on their sanctions; some could cut off relations with the IRGC.”

[Updated at 5:37 p.m. ET] Mexico's foreign ministry, explaining why Arbabsiar was denied entry into Mexico on September 28, says Mexican immigration officials blocked him because of an arrest warrant issued by the United States.

U.S. authorities arrested Arbabsiar a day later in New York, where he had flown after being denied entry into Mexico, the U.S. Justice Department has said. The Justice Department says Arbabsiar had intended to go to Mexico to guarantee final payment for an assassination of the Saudi ambassador to the United States. Arbabsiar had arranged to hire someone for the assassination that he thought was a drug cartel associate, but actually was a DEA confidential source, the Justice Department says.

"In strict compliance with domestic and international law, Mexico was able to neutralize a significant risk to Mexico’s national security, while at the same time reinforcing bilateral and reciprocal cooperation with the United States," the Mexican foreign ministry said in a statement released Tuesday. "This operation confirmed that adequate mechanisms and procedures are in place to anticipate and prevent the presence in Mexico of individuals that pose a risk to national security and interests."

[Updated at 5:03 p.m. ET] U.S. Secretary of State Hillary Clinton told reporters that the United States "will be consulting with our friends and partners around the world about how we can send a very strong message that this kind of action, which violates international norms, must be ended."

She also said the United States would consult with nations about possibly taking measures to "further isolate (Iran) from the international community."

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