"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," Justice Department spokesman Dean Boyd said.
The officials said the arrest capped a lengthy undercover operation in which the suspect had been closely monitored by law enforcement authorities. The suspect thought he was dealing with members of al Qaeda, but they were really undercover agents, one official said.
When arrested near the Capitol, the man was carrying a vest he thought was packed with explosives, but it was not dangerous, the official said.
Sergeant Kimberly Schneider, a spokeswoman for the US Capitol Police, said there was no danger to the public or members of Congress.
There have been a number of undercover operations in the Washington, D.C., area in recent years in which suspects thought they were plotting to carry out terrorism attacks, but in reality were being monitored by FBI agents and posed no danger.
There were no details on the suspect's name, his age, when he came to the United States or the charges he will face.
The officials said the case is expected to be brought by the US Attorney's office in nearby Virginia.
