Virginia Railway Express has a slasher on the loose. As of Wednesday, 21 seats on the commuter train service had been slashed within a six-week period, VRE spokesman Mark Roeber said.

At first the agency didn’t realize it had a serial vandal. Crews fixed one seat, then another on the trains that connect Northern Virginia’s outer suburbs to downtown Washington. But it kept happening — usually in the same seat in the same evening trains on the Manassas line and always the same deep gash that ran some 15 inches long, he said.