The affidavit noted that on Friday morning, Yusuf and Hussien drove El Khalifi to a parking garage "in close proximity to the United States Capitol." The document alleged that El Khalifi donned the vest containing the inert bomb and the inoperable MAC-10.
"El Khalifi told Hussien and Yusuf that he intended to use the MAC-10 automatic weapon to shoot people before detonating the bomb," the affidavit said. "El Khalifi walked alone from the vehicle towards the United States Capitol."
Agents arrested him before he exited the parking garage.
The public was never in danger, law enforcement officials told ABC News. However, officials said, the case is a sobering reminder that there remains a threat of homegrown radicals, so-called lone wolves.
"This arrest was the culmination of a lengthy and extensive operation, during which the individual was closely and carefully monitored," said Sgt. Kimberly Schneider of the U.S. Capitol Police, who said the suspect was arrested "in the area" of the Capitol building.
"The U.S. Capitol Police was intimately involved in the investigation for the duration of the operation. At no time was the public or congressional community in any danger."
"We can confirm that there has been an arrest of a suspect in Washington, D.C., in connection with a terrorism investigation," said Department of Justice spokesman Dean Boyd.
"The arrest was the culmination of an undercover operation during which the suspect was closely monitored by law enforcement," Boyd said. "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. Additional information will be forthcoming at the appropriate time."
The FBI has carried out a number of undercover terrorism sting operations in the last several years, and this one, federal sources said, was in the same mold as those executed in the past.
A number of those would-be terrorists targeted locations in Washington, D.C.
For example, in September 2011, Rezwan Ferdaus, a U.S. citizen from Ashland, Mass., and a Northeastern University physics graduate, was nabbed in an elaborate FBI sting after he told undercover officers exactly how he planned to arm "small drone airplanes" with explosives in order to hit the Pentagon and U.S. Capitol building
In April 2011, Farooque Ahmed pleaded guilty for his role in conducting surveillance to case Metro stations in Arlington, Va., in what he believed would be an attack on four Metro stations, including the Pentagon station, in a plot to kill military personnel. Ahmed was also arrested in an FBI sting operation.