What’s shocking, Mr. Seraphin said, was that 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, whom he and his team eventually surveilled.

“The license plate was either tied back to the guy or the girlfriend of the guy … but it was the same address type of thing,” he said.

“Now, at the end of the day, that makes [him] a person of interest, but it doesn’t make him the subject. … It could have been anybody associated with [the person who dropped off the devices], but it was the place to start.”

Mr. Seraphin and his team surveilled the retired airman, who lived in a Northern Virginia town house, for a couple of days and learned about his background.

Although Mr. Seraphin, who also served in the Air Force, wanted to approach the Air Force veteran and talk to him, his bureau superiors forbade him to do so before his team was removed from the case.

“I don’t know what they [eventually] did on that case, but I know that it was BS and the bombs were BS, and it seems like they had a good lead, and they could have run it down. But as far as I know, they never did,” he said. “He may still be occasionally surveilled. That’s how dumb it gets.”

Mr. Swecker, the former FBI assistant director, said he couldn’t explain why the Air Force veteran wasn’t pursued more aggressively.

“That’s a lot of good lead material there to work with. And you can peel back layer after layer using that information. I’m just astounded,” he said. “It just doesn’t add up. There’s just way too much material there to work with. … There’s just too much video, financial transactions, a car, a Metro card. There’s just too much to work with there to not know who this guy is.”