the FBI early on linked the bomber to a D.C. MetroRail SmarTrip card. The card indicated that the person got off a train at a Northern Virginia stop after planting both devices on Jan. 5.

“So they tagged the entrance time and the exit time to that card to that guy. And then they found out who bought the card. And the guy who bought the card was not the guy who was using it,” Mr. Seraphin said.

“The card had never been used before. It was bought a year prior by a retired chief master sergeant in the Air Force, and he was a security contractor. So he held a security clearance.”

The FBI had surveillance video that showed the person entering a car with a visible license plate after exiting a Metro stop in Northern Virginia.

“So what they did is they tied whoever the person was that dropped the bombs with [surveillance] cameras all the way through the train and getting into a car with that license plate,” Mr. Seraphin said.

He said the vehicle plate was traced back to the address of the Air Force veteran.