Concluding that damage to two new combat helicopters at the Boeing Co. plant was "a deliberate act," federal authorities said yesterday they had launched a criminal probe and offered a reward in the hunt for suspects.

U.S. Attorney Patrick L. Meehan said he had assigned a prosecutor to work with criminal investigators from the Defense Department, who now say they believe damage to the Chinook choppers was an act of vandalism.

Agents with the Defense Criminal Investigative Service, which is leading the probe on behalf of the Army, circulated flyers offering employees a $5,000 reward. The helicopters cost $20 million to $30 million apiece and are part of a 458-Chinook contract for Boeing, which shut down operations Tuesday but resumed production yesterday.

"We have determined that this was a deliberate act and not an accident," said Kenneth S. Maupin, the Defense Department's lead investigative agent, during a news conference with Meehan outside the battleship-gray facility where authorities toured the Chinook assembly line minutes before addressing the media.

The announcement further intensified suspicion that the potential culprit may be a member of Boeing's 5,200-member workforce, a combination of unionized and contract employees who work on the sprawling campus south of Philadelphia International Airport in Delaware County.

None of the damaged helicopters - Chinook CH-47Fs - has been deployed overseas. Boeing officials said they believed the problems detected this week were isolated to just two aircraft and did not affect others.

Older Chinook helicopters are in use in Iraq and Afghanistan. They are critical in such combat zones because their tandem rotors help them reach high altitudes for gear drops and other maneuvers.

While the company has said the damage to the helicopters is not irreparable, Meehan said the incident raises safety concerns for soldiers, economic issues for Boeing, and workforce issues for Boeing employees, who were frozen out of work while production shut down.

"There are soldiers in a very short period of time that will be taking this kind of an aircraft into harm's way," Meehan said.

Meehan said he requested the tour of the facility, but would not say what he saw. Damage was said to include severed wires on one aircraft in one case, and a propeller part found in a spot where it did not belong in the other case.

"We don't customarily, during the course of an investigation, frankly, even confirm the existence of an investigation," Meehan told reporters during an afternoon huddle outside the building. "But we are here on site because it's important to see it firsthand."

DCIS, whose agents are involved because the helicopters were being produced as part of a major Army contract with Boeing, also set up a hotline (267-228-2782) for tips - even anonymous ones.

Maupin's team of 10 investigators is working with Meehan's office.

Work on the Chinook assembly line was shut down Tuesday, but employees reported back on regular shifts for the first time yesterday, said Boeing spokesman Jack Satterfield.

Efforts to reach John DeFrancisco, president of Local 1069 of the United Auto Workers, were unsuccessful yesterday. A day earlier, DeFrancisco had said employees were "very concerned" about the investigation. His union represents 1,640 production and maintenance workers.

DeFrancisco said he had received reports from the facility that wiring on one chopper appeared to have been "hacked."

Officials offered no motive yesterday.