A MOROCCAN immigrant was arrested as he tried to carry out an Al-Qaeda suicide bombing at the US Capitol, under the watch of the FBI in a sting operation, officials said.

The FBI said it conducted the undercover operation as part of a longstanding terrorism probe of the suspect, who was arrested blocks from the Capitol building as he planned to detonate what he believed to be live explosives in a vest he wore.

Both the explosives and a MAC-10 automatic gun were given to him by undercover federal agents, who had first rendered them inoperable, according to court documents.

Because of that, the incident “posed no threat to the public,” the Federal Bureau of Investigation said in a statement.

The Justice Department identified the man as Amine El Khalifi, a 29-year-old immigrant from Morocco living illegally in the US state of Virginia, which borders the capital Washington.

“Amine El Khalifi sought to blow himself up in the US Capitol Building,” US Attorney Neil MacBride said in a statement.

“El Khalifi allegedly believed he was working with Al-Qaeda and devised the plot, the targets and the methods on his own,” the statement said.

According to court documents, Khalafi attended a meeting in Arlington, Virginia, in January 2011 which included a “confidential human source” who reported to the FBI that Khalifi wanted to join an effort to stage attacks on US soil.

An affidavit says Khalifi “sought to be associated with an armed extremist group,” and on December 1, was introduced to a man called “Yusuf” who was in fact an undercover agent.

The Justice Department said Khalifi had originally considered several different targets, including US military offices, a synagogue and a restaurant frequented by US military officials, but eventually settled on the US Capitol.

He spent nearly a month scoping out the building, venturing inside several times to conduct surveillance and asking a man he thought was an Al-Qaeda operative — but who was in fact a US undercover agent — to remotely detonate the bomb if he encountered problems with security officers.

The Justice Department said undercover agents were with Khalifi on Friday when they drove to a parking garage, where the suspect took possession of an automatic MAC-10 pistol “and put on a vest containing what he believed to be a functioning bomb.”

The statement said that unbeknownst to Khalifi, “both the weapon and the bomb had been rendered inoperable by law enforcement.”

He then walked toward the Capitol “where he intended to shoot people and detonate the bomb,” but Khalifi was arrested and taken into custody before exiting the parking garage, the department said.

Khalifi made an initial appearance Friday before a federal judge, where he was charged with attempting to use a weapon of mass destruction against US property. He faces a maximum penalty of life in prison if convicted.

Authorities briefed US lawmakers — who gather in the Capitol building to debate and vote on legislation — on the arrest.

“The brazen nature of this plot — targeting the US Capitol building with the aim of killing innocent people and desecrating a symbol of our democracy — is disturbing,” Republican Senator Susan Collins said in a statement.

“This plot appears to be yet another example of radicalized extremists attempting to attack Americans from within our borders.”

Collins, the ranking Republican on the Homeland Security Committee, mentioned figures that show a sharp rise in thwarted terrorism plots in recent years.

According to the Congressional Research Service, between May 2009 and February 9, 2012, arrests were made in connection with 36 homegrown plots by Americans or legal permanent US residents, compared to 21 such plots between September 11, 2001 and May 2009.

Friday’s arrest was startling because it marked a plot by a US-based suspect apparently intent on blowing himself up, whereas several previous sting operations foiled plots that did not necessarily involve suicide attacks.

While US authorities have been on alert to terrorism plots after the 9/11 attacks, in which terrorists flew hijacked planes into the World Trade Center and the Pentagon, “I think we have this false sense that a suicide bombing... is unlikely to happen here,” said Frances Townsend, former homeland security advisor to president George W. Bush.

A White House spokesman said President Barack Obama was informed Thursday of the FBI’s plan to make an arrest.             AFP