WASHINGTON (AP) -- An investigation of a firefighting helicopter crash that killed nine people two years ago has revealed multiple layers of failures in government safety oversight and by the company that leased the helicopter to the U.S. Forest Service, accident investigators said Tuesday.

Carson Helicopters of Grants Pass, Ore. deliberately understated the helicopter's weight by more than 1,000 pounds in order to make it appear the aircraft could safely carry a heavier payload, investigators said. That helped the company win a Forest Service firefighting contract, they said.

The helicopter was more than 500 pounds over the maximum weight at which it was capable of lifting off when it crashed near Weaverville, Calif., on Aug. 5, 2008.

The Federal Aviation Administration and the Forest Service missed repeated opportunities to catch the weight miscalculations, investigators said. Two months after the accident, the FAA office in charge of overseeing Carson received letters from two pilots expressing concern that the company was miscalculating helicopter weights, they said.

However, FAA dismissed the allegations and didn't provide the letters to NTSB until about a year later after the investigators made a general request for documents related to oversight of Carson, investigators said. FAA was a party to the accident investigation and its inspectors were aware of the investigation, they said.

An FAA spokeswoman declined to comment.

FAA officials have said they don't have the authority to regulate safety of aircraft leased or owned by other federal agencies or state and local government agencies as long as those aircraft are dedicated exclusively to government operations.

The accident reveals multiple layers of failures and missed opportunities, said National Transportation Safety Board Chairman Deborah Hersman said. She likened safety oversight of government aircraft to orphan children.

They have no parent and no one wants to be responsible for them, Hersman said.

The board is meeting to determine the cause of the accident and make safety recommendations.

Besides the accident helicopter, Carson also underestimated the weight of about eight other helicopters in an identical fashion went submitting bids to the Forest Service, investigators said.

Carson also encouraged pilots to use a system for calculating a helicopter's weight on liftoff that eroded safety margins, investigators said.

The helicopter weighed 19,008 pound when pilots tried to take off from a rugged mountaintop clearing. But the maximum weight to lift off at full power with no margin to spare was 18,445 pounds, they said. If Forest Service guidelines -- which include a safety margin -- had been followed, the weight shouldn't have exceeded 15,840 pounds, investigators said.

The Sikorsky S-61N helicopter was airborne less than a minute when the rotor began to slow, it clipped a tree and fell into the forest. It was carrying firefighters from the front lines of a stubborn wildfire in the Trinity Alps Wilderness.

Seven firefighters, the pilot and a Forest Service safety inspector were killed. The co-pilot and three firefighters were injured. Survivors told the board that they were unable to unbuckle their seatbelts and had to wiggle out of them in order to escape the downed helicopter before it was consumed in a post-crash fire.

The Forest Service and the FAA failed to sufficiently oversee Carson and detect fatal errors and discrepancies that should have been identified, and corrected, before the accident, Hersman said.

Twenty-three federal agencies, including the Forest Service, operate 1,632 nonmilitary planes and helicopters, according to the General Services Administration. Some are owned and maintained by the government, while others are leased from private companies such as Carson. Hundreds more are operated by state and local governments.

Government agencies have policies that leased aircraft should come from companies that have been certified by the FAA, according to the GSA. But FAA limits its inspections and oversight to the portions of the leasing companies' operations involving the public, excluding aircraft dedicated to government use, investigators said.

Part of the reason for that is that FAA has no expertise in many of the kinds of operations that government aircraft are involved in, such as firefighting, investigators said.

Fatal accidents involving government aircraft are commonplace. A 2001 study by the board said there were 341 accidents involving nonmilitary government aircraft between 1993 and 2000. Among accidents in the last two years:

--A Forest Service plane conducting an aerial survey of tree defoliation in southwestern Pennsylvania in June struck a light post while trying to land near Lock Haven, Pa. The pilot and two Forest Service employees were killed.

--A California Fish and Game Department helicopter surveying deer collided with power lines near Fresno, Calif, in January. The pilot and three passengers were killed.

--Also in January, a U.S. Fish and Wildlife Service helicopter on a repositioning flight crashed northwest of Corvallis, Ore., killing the pilot and a passenger.

--A New Mexico State Police search and rescue helicopter that had just retrieved a lost hiker crashed into a hillside near Santa Fe in June 2009. The pilot and the hiker were killed; a patrolman acting as a spotter was seriously injured.

--Two pilots and a passenger were killed in April 2009 when an air tanker leased by the Forest Service crashed near Stockton, Utah, in rain and fog.

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