The federal government said yesterday that it has solved the three-year-old mystery of a small plane crash in Alabama that, at least initially, appeared to have been the result of an in-flight collision with an unexplained object.

The nighttime crash in October 2002 of the single-engine Cessna cargo plane killed the pilot, the plane's only occupant, and launched UFO and government-conspiracy theories on Web sites, pondering what it might have collided with. Red scuff marks were found on pieces of the wreckage after it was pulled from a swamp.

Some pilots theorized that the Cessna was struck by a stray unmanned drone, many of which are painted red or orange, that are flown by the military at a nearby Air Force base. The investigation so frustrated the dead pilot's sister that she spent weeks combing Big Bateau Bay for clues.

After taking the unusual step of moving the aircraft to Washington and sending the red paint marks to four laboratories for testing, the National Transportation Safety Board concluded that the pilot became disoriented while flying through cloud layers and crashed the plane probably after seeing a large plane coming near him.

Tests from the labs, one at the FBI, found that most of the red-marked pieces were similar to materials inside the cargo plane, including baseball caps, audiotape packaging, a fire extinguisher, a tow bar and a Priority Mail envelope. The Wright Patterson Air Force Laboratory said that red color on the wreckage did not match the polyurethane coating on the exterior of the drone it used from Eglin Air Force Base.

"Wreckage examinations . . . revealed no evidence of an in-flight collision or breakup, or of external contact by a foreign object," the NTSB's report said. The red scuff marks on the wreckage did not appear in a pattern that suggested a collision, the report said.

The safety board's simulation of the accident revealed that the pilot, Thomas J. Preziose, was flying between cloud layers when he was notified of a FedEx DC-10 flying in the area. Even though the DC-10 was 2,400 feet higher and more than a mile away, it probably appeared much closer to the pilot, the report indicated. The pilot's "view of the DC-10 moved diagonally across the windscreen from his left to straight in front of the Cessna while [appearing to triple] in size" in just seconds, the report said.

If a pilot is distracted and does not have a view of the horizon, as in this case, it is easy to become disoriented, said John Clark, head of NTSB's Office of Aviation Safety. "It's quite insidious if you're between layers of clouds; you can get screwed up," he said. "If you start moving your head or moving up and down, you can screw up your inner balance."

Still, a lawyer who said he represents two sons of the pilot, said he does not believe the safety board's conclusion and that he plans to press ahead in federal court with a lawsuit that blames the weather, air traffic control and other factors.

"I don't think the board's got it straight," Tony B. Jobe said. "The plot is thickening."