The FBI paid $3,100 last summer for a 1976 Chevelle identical to the one driven by former Chilean ambassador Orlando Letelier when he was assassinated and then blew up the car with a bomb identical to the one Chilean secret police agent Michael V. Townley said he used to kill Letelier.

Townley, the government's chief witness against three anti-Castro Cubans convicted Wednesday in connection with the Letelier slaying, showed the FBI how to build the remote-controlled bomb and how he placed it in the undercarriage of the car beneath the driver's seat, according to government prosecutors in the case.

The second car was blown up July 17 at a demolition field at the Quantico Marine Base. The blast was filmed by nine cameras from different angles so investigators could study in extraordinary, slow-motion detail how Letelier's assassination was carried out, according to Assistant U.S. Attorneys E. Lawrence Barcella Jr. and Eugene M. Propper.

Barcella said yesterday that when the experiment was done the prosecutors still did not know what kind of defense the then-prospective defendants might raise. There was some feeling, he said, that an argument might be made that Townley was lying when he said he built and planted the bomb in Letelier's Chevelle Malibu Classic.

A 17-minute film was made of FBI agents attaching the bomb to the second car as it was hoisted on a hydraulic lift at FBI headquarters and then as it was blown up at Quantico. The explosion and aftermath, which took 3.5 to 4 seconds by the time debris showered to the ground, was expanded into 12 minutes of film to show each segment of the car blowing up.

As it turned out, the results of the blast were remarkably similar to the explosion that killed Letelier and a colleague of his at the Institute for Policy Studies, Ronni K. Moffitt, as they drove along Washington's Embassy Row.

The hoods and roofs of both cars buckled into a pyramidal shape and the molding along the driver's side of the car was either ripped off or left dangling from the wreckage, pictures of the two cars show.

The film of the second explosion was never used at the Cubans' trial because defense lawyers did not contest Townley's assertion that he built and planted the bomb and because U.S. District Court Judge Barrington D. Parker limited prosecutors in the amount of evidence they could introduce to depict the blood and gore of the murders.

Nonetheless, Barcella said the film, whose cost is undisclosed, has served a variety of purposes.

Barcella said he had talked about the Letelier explosion with Michael Moffitt, who was riding in Letelier's car and slightly injured when it blew up. "But until I saw the (second) car blow up I never realized the terror of the situation," the prosecutor said. "That Michael Moffitt survived that blast is miraculous."

Propper said investigators and prosecutors watched the blast on a television screen in a concrete bunker at Quantico 50 yards from the blast. But even from the safety of the bunker, Propper said that when the car exploded "it was like someone slugged you."

Barcella said the film may be used in future trials involving the Letelier killing and was sent as part of a package to Chile seeking the extradition of three secret police agents to stand trial in the U.S. for the murder of Letelier and Moffitt. In addition, Barcella and Propper said the FBI now has a first-rate training film showing how a car explodes.