Mr. Young, a United States citizen who lives in Fairfax, Va., joined the Washington Metropolitan Transportation Authority’s police force in 2003. The authority, which said he was dismissed after his arrest on Wednesday, would not say where he was assigned or what kind of work he did.

An undercover law enforcement officer who befriended and monitored him in 2011 and 2012 reported that Mr. Young had spoken of committing violence against the F.B.I., including using gasoline to set agents’ cars on fire and kidnapping and torturing an agent who had interviewed his family members and co-workers, according to an affidavit by an F.B.I. agent, David Martinez.

The affidavit says that Mr. Young knew he was probably under surveillance and that he took steps to evade it, like using disposable phones. But it said he did talk to the undercover officer about sneaking weapons into a federal courthouse, and stockpiling weapons.

Mr. Young came under F.B.I. scrutiny in 2010 because of his acquaintance with Mr. Chesser, and then he and the undercover officer met several times in 2011 with Mr. Khalifi and discussed violent jihad, the affidavit says.