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Authorities arrested and charged Friday a Virginia man allegedly on his way to the U.S. Capitol for what he thought would be a suicide attack on one of the nation's most symbolic landmarks.

The federal criminal complaint against the suspect identifies him as Amine El Khalifi, a 29-year-old Moroccan citizen who has been living in the United States illegally since 1999 after his visa expired. He was nabbed following a lengthy investigation by the FBI, initiated after he allegedly expressed interest in conducting an attack. Court documents say he came onto the radar screen in early December after he told an undercover agent about an earlier plan to bomb a northern Virginia building.

According to charging documents, he first entered the country that year on a tourist visa, which expired and was never renewed. Khalifi was charged Friday in U.S. District Court in northern Virginia with attempting to use a weapon of mass destruction.

The suspect allegedly weighed hitting various targets ranging from a military installation to synagogues to a Washington restaurant before settling on the Capitol.

The man thought undercover FBI agents assisting him in his plot were associates of Al Qaeda. He purchased bomb materials including jackets, nails and glue in preparation for an attack. He even conducted a test explosives demonstration in a quarry.

When he was arrested Friday in Washington, he was carrying with him a vest that he had been led to believe was packed with explosives, but the material inside was not actually dangerous, Fox News was told.

A short time earlier, Khalifi had been praying at a mosque in the Washington area. His destination was Capitol Hill.

The public was never in danger, as he had been under constant surveillance for some time, officials said. The FBI provided the suspect with a disabled gun during their ongoing operation, Fox News has learned.

The U.S. Capitol Police, in a statement that confirmed the arrest but provided few details, said the suspect had been "closely and carefully monitored."

A senior source involved with law enforcement at the Capitol also told Fox News the investigation was "all very controlled." The source said the U.S. Capitol Police was involved with the FBI and other agencies in tracking the suspect "not more than a year."

A former landlord in Arlington said he believed El Khalifi was suspicious and called police 18 months ago.

Frank Dynda said when he told El Khalifi to leave, the suspect said he had a right to stay and threatened to beat up Dynda. Dynda said he thought El Khalifi was making bombs, but police told him to leave the man alone. Dynda had El Khalifi evicted in 2010.

El Khalifi had several men staying with him and based on packages left for him, Dynda said, it appeared that he was running a luggage business from the apartment, although Dynda never saw any bags.

"I reported to police I think he's making bombs," Dynda said. "I was ready to get my shotgun and run him out of the building, but that would have been a lot of trouble."

On Capitol Hill, lawmakers in leadership positions had been briefed on the investigation, though rank-and-file members did not appear to have prior knowledge of the case.

Rep. Emanuel Cleaver, D-Mo., called the plot a "stark reminder" of the dangers Americans face.

"I think it will encourage more of us to take the tunnel. ... Maybe we have to walk around with a little higher level of paranoia," Cleaver told Fox News.

Sites in Washington have long been a target for terrorists, especially self-radicalized extremists caught in FBI stings.

In September, a Massachusetts man was arrested for allegedly plotting to fly bomb-laden model planes into the Pentagon and U.S. Capitol. FBI agents claiming to be associates of Al Qaeda provided 26-year-old Rezwan Ferdaus with what he thought was explosive material for the remote-controlled planes.

Nearly a year earlier, a Virginia man was arrested for trying to help Al Qaeda plan multiple bombings against Washington's Metrorail system. For months, 34-year-old Farooque Ahmed of Ashburn, Va., had been meeting and discussing "jihad" with individuals he thought were affiliated with Al Qaeda, but in fact he was meeting with FBI agents.

In the past year alone, at least 20 people have been arrested in the United States on terrorism-related charges, according to the Senate Select Committee on Intelligence.

"Most of the arrests" have involved "lone wolves," radicalized online and able to use the Internet to build bombs, FBI Director Robert Mueller told the Senate committee last month.

At the time of Ahmed's arrest in October 2010, the U.S. Attorney for the Eastern District of Virginia, Neil MacBride, said the case showcases "our ability to find those seeking to harm U.S. citizens and neutralize them before they can act."

Fox News' Catherine Herridge, Trish Turner, Chad Pergram and the Associated Press contributed to this report.