RIDLEY PARK, Pa. —  Investigators have concluded that two military helicopters were vandalized on the production line at a Boeing factory near Philadelphia, the Defense Department said Thursday.

Federal officials were handing out fliers to workers at the Boeing Rotorcraft Systems plant in Ridley Township, Pa., in which they offered a $5,000 reward for information leading to the identity of whoever vandalized the two H-47 Chinook helicopters.

"We have determined that this was a deliberate act and not an accident," said Ken Maupin, resident agent in charge of the Philadelphia area office of the Defense Criminal Investigative Service.

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

A production line at the plant has not been fully functional since Tuesday when two workers found what the company called irregularities in the helicopters. U.S. Rep. Joe Sestak has said he was told that wires that appeared to be broken or severed were found in one helicopter and that a suspicious washer was found in a second.

U.S. Attorney Patrick Meehan said Thursday that he was comfortable with the conclusion the damage was done deliberately.