Arson was the cause of the fire that killed two Navajo tribal policemen and destroyed their police panel trucks, according to the testimony Tuesday of an FBI laboratory examiner.

Ronald Duncan of the FBI laboratory in Washington, D.C., testified that shortly after the killings of Officers Roy Lee Stanley and Andy Begay in December he was part of a team that investigated the site where the men's trucks and bodies were discovered.Earlier a medical examiner said both officers died in the fire, although they had been shot first.

Duncan testified during the trial of those accused of committing the murders - Thomas Cly, Marques Atene, Vinton Bedonie and Ben Atene Jr. It is being held in U.S. District Court because the killings happened on the Navajo Indian Reservation, which is under the jurisdiction of the federal government.

The trucks were 13 feet apart at the closest, so it seems improbable that both could have burst into flames accidentally at the same time or that flames from one could have ignited the other, Duncan said.

In a situation like that, "My feeling is definitely it's arson," he said.

He said the fire was obviously very hot and some of the vehicles' pot metal had melted. "There probably was an accelerant used," such as gasoline, he said.

However, he could not find traces of gasoline, since it is completely consumed when it burns.

The jury Monday afternoon watched a

videotape of the removal of Stanley's and Begay's bodies. In one wide-angle view

shown on the video screen, an object on the white body bag looked like a black ball.

"I believe that was Officer Begay's head," said former FBI Special Agent Joseph Kay Brooks.

Assistant U.S. Attorney Stewart Walz had told the jury this was the sequence: Stanley discovered an illegal beer party at a crossroads near Goulding's, San Juan County, close to Monument Valley; he was involved in a fight, then was shot with his own pistol; Begay arrived later that night, was confronted with Stanley's pistol and shot; both officers were driven to Copper Canyon, where they burned to death when their police trucks were torched.

The defense denies guilt, contending that the wrong people were arrested.

U.S. District Judge J. Thomas Greene stood beside Brooks, who was in the witness chair, so he could watch the video monitor. It was turned toward the jurors.

Brooks - who retired from the FBI on July 1 after 24 years - described the action as an edited videotape was shown of the removal of the bodies. The view was not close up, so grisly details weren't visible. The tape was soundless.

Both bodies were in Begay's four-wheel-drive vehicle, he said. When they were removed on Dec. 7, the truck's rear door was fused shut, so an agent had to saw through the hinges.

Brooks said both officers' revolvers were on the front floor of the other police panel truck. They were blackened and both had the grips burned off.

Earlier in the day, defense lawyers laid out their case, saying some of the defendants were at a religious ceremony, called the "Blessing Way" for Albert Atene, who was a victim of a hit-and-run accident in September 1987.

Thomas Cly was drunk and not at the scene of the crime, said his lawyer, Ed Brass.

Lawyers for defendants focused on the fact that some witnesses have changed their stories, saying jurors should look out for prior inconsistencies. They said the charges could have been based on gossip.

Although 49 people were somehow involved in the case, and beer cans and beer bottles were found from the scene where the officers were killed, none of the bottles or cans carried the defendants' fingerprints, Brass said.

One witness is expected to testify for the government that he accompanied the trucks to Copper Canyon, and one of the details he would recite involves a man stopping and relieving himself.

Bureau of Indian Affairs agent Robert Sombrero, a Navajo from Tuba City, Ariz., testified Tuesday that sometime more than 38 hours after the time the incidents are supposed to have happened in Copper Canyon, he photographed footprints and a wet spot where someone had relieved himself at a gate six miles from the trucks.