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, at a news conference outside the plant.
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.
The Chinook is the Army's workhorse aircraft and is used to transport troops and supplies. Boeing is currently producing new Chinooks for the Army, as well as updating older models.
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.
"You have a large number of workers here at this point and one or more that was the problem," Maupin said. "The majority of the people here are hardworking, loyal Americans, and many of them are veterans. They want to find (those responsible) as much as we do."
U.S. Attorney Patrick Meehan said that he was comfortable with the conclusion the damage was done deliberately.
"There are federal statutes that would implicate anybody who intentionally interfered with a mechanism like this that is being introduced into a theater of war," Meehan said.
Jack Satterfield, a company spokesman, said Boeing had no comment about the investigators' determination and would not be releasing information about what was found.
"We're leaving that to the investigative agencies," Satterfield said.
John DeFrancisco, president of United Auto Workers 1069, said that workers take pride in making aircraft for the military and that he believes information would have surfaced about what happened without a reward.
"We're very, very concerned and we're very, very upset," DeFrancisco said. "We're upset that, if this is deliberate, it affects all of us."