The FBI is investigating the cause of several puncture holes found near the landing gear of three US Airways aircraft.

The holes, ranging in size from a pencil point to a dime, were found on a Boeing 737 and two Airbus 320s during routine inspections earlier this week. Two of the planes were at the airline's hub airport in Charlotte and one was in Orlando.

FBI spokesman David Martinez said the agency was assessing whether the damage was the result of regular operations or an intentional act.

"That's why we've opened up an investigation. We are trying to determine those various issues," Martinez said.

The holes posed no safety danger and the repaired planes are back in service, said US Airways spokesman David Castelveter.

US Airways, which is operating under Chapter 11 protection for the second time in two years, won approval from the bankruptcy court last week to cut workers' pay and benefits by 21 percent through February.

The Federal Aviation Administration automatically steps up surveillance of maintenance operations of airlines in bankruptcy. It also pays closer attention to carriers seeking pay cuts from workers. The aircraft holes were found by airline employees performing standard preflight inspections.

Castelveter said the airline has contacted the FBI in the past to investigate unexplained damage to its aircraft. "There are a variety of potential reasons for the damage," he said.

The FBI and US Airways employees familiar with the case said that gravel kicked up during landings may have penetrated the aircraft's belly. Martinez said that gravel was one possibility the FBI was investigating.

Industry observers said that disgruntled employees have rarely been known to harm aircraft.

Far more common is for employees to slow down their work, adding paperwork or spending extra time on maintenance to keep planes in the hangars longer.

Consultant Mike Boyd of the Boyd Group said that along with US Airways employees, non-company workers such as fuel operators and airport personnel have access to parked planes.

"It's unlikely a US Airways employee would do something like this," Boyd said.