Chinook helicopters vandalized at Boeing plant in Pa.

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 as it offered a reward in the case.

Federal officials were handing out fliers to workers at the Boeing Rotorcraft Systems plant listing a $5,000 reward in the damaging of 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.

Associated Press Kimberly Hefling in Washington contributed to this report.