Since 1886, the A.T.F.’s laboratory division has been spread across those three places. Its modest staff includes chemists, forensic biologists and scientists who analyze evidence each year from thousands of fires, explosions and other crimes. In 2017, A.T.F. labs helped close 314 explosives cases.

“I don’t know that there’s a better bomb lab in the country,” Brad A. Galvan, a former A.T.F. agent who ran the bureau’s explosives unit in San Diego until 2017, said of Beltsville.

To go from a blast site to Beltsville or a similar facility, a deactivated bomb — or parts of it — is transported to one of the A.T.F. labs. Stored at the facilities in bunkers, the samples can be road maps to suspects.

“We’re going to dissect that device down to its lowest common denominator,” Mr. Galvan said. “Anything that’s used and recovered, we’re going to try and identify. It’s very manpower-intensive.”

Each piece, down to the type of tape used to hold a bomb together, helps build a profile of the suspect. Shrapnel types, a certain kind of PVC piping, spit on the back of a stamp — any could lead to the right security footage or hardware store.

“Sometimes these areas are remote enough that you’ve only got one Lowe’s or one Home Depot in the whole area,” Mr. Klapec said.