The FBI has arrested a suspect who was en route to the U.S. Capitol allegedly to detonate a suicide bomb, USA TODAY's Kevin Johnson reports.

Update at 4:50 p.m. ET: A bail hearing has been set for 29-year-old Amine El Khalifi of Alexandria, Va., an unemployed Moroccan who authorities said is in the United States illegally. He arrived when he was 16 and overstayed his visitor's visa.

According to a counterterrorism official, El Khalifi "expressed interest in killing at least 30 people and considered targeting a building in Alexandria and a restaurant, synagogue and a place where military personnel gather in Washington before he settled on the Capitol after canvassing that area a couple of times," the Associated Press writes. During the year-long investigation, El Khalifi detonated explosives at a quarry in the capital region with undercover operatives. He is not believed to be affiliated with al-Qaeda, officials said.

Update at 2:41 p.m. ET: "We can confirm that there has been an arrest of a suspect in Washington, D.C., in connection with a terrorism investigation,'' Justice spokesman Dean Boyd said, adding that the arrest was the culmination of a lengthy undercover operation "during which the suspect was closely monitored by law enforcement.''

"Explosives the suspect allegedly sought to use in connection with the plot had been rendered inoperable by law enforcement and posed no threat to the public,'' Boyd said.

Update at 2:13 p.m. ET: The man, believed to be in his 30s, was apprehended while on his way to allegedly carry out the attack with a device that undercover agents had previously rendered inoperable, said a federal law enforcement official who was not authorized to comment publicly.

The official says the suspect was the target of an undercover investigation for about two months and that the public was never in any danger.

Update at 2:04 p.m. ET: A U.S. law enforcement official said the person arrested was canvassing the U.S. Capitol with violent intentions, the Associated Press reports. He was not believed to have any known connections to al-Qaeda, the AP reports.

U.S. Capitol police say their officers were also involved in the arrest.

Update at 1:59 p.m. ET: U.S. officials said the arrest "was the culmination of a lengthy and extensive operation during which the individual was closely and carefully monitored," WUSA TV reports.

Update at 1:57 p.m. ET: The suspect was arrested at a steakhouse on Constitution Avenue near the Capitol, CNN reports.

Update at 1:52 p.m. ET: The suspect was arrested near the Capitol building, the Associated Press reports.

Original post: NBC Justice correspondent Pete Williams says the suspect, described as being in his early 30s, had been under observation for several months.

Williams says the suspect was arrested by the FBI after receiving a vest that he thought contained explosives, but were actually inert materials.

Fox news reports that the suspect was carrying the vest when he was arrested in Washington.

NBC and Fox report that the suspect thought that the FBI agents who were assisting him were associates of al-Qaeda.