Two military helicopters were vandalized on the production line at a Boeing factory near Philadelphia, the Defense Department said Thursday as it offered a reward.

Federal officials handed out fliers to workers at the Boeing Rotorcraft Systems plant listing a $5,000 reward for information leading to whoever damaged the two H-47 Chinook helicopters.

“We have determined that this was a deliberate act and not an accident,” said Ken Maupin, an agent with the Defense Criminal Investigative Service, at a news conference outside the plant.

He said 10 agents were investigating, but he would not comment on specifics or what led to the determination that it was vandalism.

The Chinook is the Army’s workhorse aircraft and is used to move troops and supplies. Boeing is producing new Chinooks for the Army, as well as updating older models. The military has not grounded any helicopters now in use.