From the outset, the crash didn't make sense to air safety investigators. The plane, a Cessna 152, appeared to have been flown into the ground at high speed and apparently under full control. It had none of the markings of an accident.

The crash occurred at about 11:30 a.m. on a clear November day and killed Rene Carlos Vos of Arlington and Gerda Ruhnke of Reston.

Vos was a 30-year-old gun store owner who had been granted immunity last year to testify before a federal grand jury. Ruhnke, 48, a flight instructor, was well known in local aviation circles as an excellent pilot and instructor.

It is not clear whether there is a relationship between the Nov. 19 crash and Vos' role in the grand jury investigation. Sources have confirmed that the FBI is investigating the crash, something it rarely does unless there is suspicion of a federal crime.

FBI spokeswoman Barbara Wallace declined to comment about the investigation.

The crash was unusual, according to aviation specialists, because the plane was apparently flying under control and developing power when it smashed into the driveway of a house five miles east of Warrenton in Fauquier County. National Transportation Safety Board investigators say they have found no signs of mechanical or structural failure.

The Cessna 152 is a two-seat, dual-controlled trainer with a small 110-horsepower engine. Aviation experts said the plane is extremely stable and would not crash in a nose-down, wings-level attitude by accident.

In virtually all nose-down crashes, the aircraft has been stalled and has gone into an uncontrollable spin, or rotation, according to aviation experts. Investigators who examined the wreckage of Ruhnke and Vos' plane found that it was not rotating when it struck the ground. In addition, they found that the plane was traveling at high speed, faster than would be likely in a stall-spin accident.

Investigators have looked into the possibility of sabotage, sources said, but have found no evidence of explosives or defective control mechanisms aboard the plane.

Autopsies of Vos and Ruhnke were incomplete because their bodies were badly burned, authorities said. For example, it was impossible to determine whether Ruhnke had suffered a heart attack or seizure. Medical tests showed that Vos was healthy at the time of the crash, and there was no indication of drug or alcohol impairment on the part of either of them, authorities said.

Lin Clayberg, Ruhnke's partner in Dulles Air Service, the flying club that owned the plane, said Ruhnke could have chosen either of two Cessna 152s for the flight. There was little chance anyone could have known in advance which plane was to be flown, Clayberg said.

Aviation specialists said it might be possible to sabotage the plane with an elaborate, remote-controlled device implanted on or near the aircraft's control cables, but they said a saboteur would have needed to remove the rear bulkhead of the plane, behind the seats, to gain access to the cables.

The airplane was based at Dulles International Airport, where it was kept at an outdoor ramp, and airport employees said they had not seen anyone in the plane before the flight.

There was no indication of trouble in Ruhnke's last radio communication with air traffic controllers at Dulles, where the plane took off about 30 minutes before the crash, Clayberg said.

Federal investigators say a murder-suicide by Vos is the most plausible theory -- that the 6-foot-1, 195-pound Vos overpowered the 5-foot-4, 130-pound Ruhnke.

Friends of both victims contend that neither would have committed a murder-suicide.

At the time of the crash, Vos was trying to open a second gun shop in Chantilly, with an indoor firing range, according to Anthony Speros Makris, who said he had known Vos since they were high school students in Montgomery, Ala., and who was one of his closest friends in Washington. Makris said that Vos had become engaged recently, had bought a new four-wheel-drive vehicle for hunting trips and had just installed a telephone in it.

"There's not much else to say except they had some sort of trouble" in the plane, Makris said. "What happened will probably never be known."

Likewise, Ruhnke's friends said she was an unlikely person to commit suicide. She was doing well in her business, had recently bought a Porsche automobile and, according to her business partner Clayberg, "loved life."

"It just doesn't compute," said Philip O'Brien, chief pilot for Dulles Air Service and a former project safety officer with the Air Force. "An airplane just doesn't drop, nose first, into the ground . . . . You'd have to continuously press on the yoke" which controls the angle of flight.

The impact of the crash "fractured" the Cessna's engine into several pieces, according to safety board investigator Al Dickinson, and dug a crater nearly a foot deep in the ground. "Engines are pretty solid. They don't often break apart," Dickinson said.

He said the force of impact was "unusually destructive," even for airplane crashes.

Makris said he identified Vos by his $7,500 Rolex President watch and "boot heels, belt buckles" -- things that he said he knew belonged to Vos.

It was Vos' first flight lesson with Ruhnke, and their first meeting, according to Clayberg. He had been referred to Dulles Air Service by another of Ruhnke's students, Clayberg said.