LOS ANGELES (AP) _ An FBI photographic analyst who studied film of the ''Twilight Zone'' movie accident said Tuesday he couldn't tell if debris hit the tail rotor of the helicopter that crashed and killed three actors.

Gerald Richards, testifying at the manslaughter trial of director John Landis and four other filmmakers, made the comments during cross-examinatio n by defense attorneys.

''Were you able to form a conclusion about what caused this accident?'' asked attorney Harland Braun.

''No,'' said Richards.

The witness had testified under prosecution questioning that he was able to see debris flying toward the helicopter as it flew over explosions in a movie set Vietnamese village.

But he said he could not see debris hit the rotor when it entered the fireball of a large explosion, when it was engulfed by the fireball or when it emerged.

Actor Vic Morrow, 53, Myca Le, 7, and Renee Chen, 6, were killed when the chopper crashed on top of them as Morrow carried the children across a stream.

Landis, associate producer George Folsey, production manager Dan Allingham, special effects coordinator Paul Stewart and pilot Dorcey Wingo are charged with involuntary manslaughter in the deaths.

Outside court, defense attorneys said Richards' testimony was helping them disprove the prosecution theory that flying debris caused the crash. They contend heat caused an unforseeable ''delamination'' or peeling apart of the rotor's skin.

But Deputy District Attorney Lea Purwin D'Agostino said Richards had shown that debris was definitely flying in the vicinity of the helicopter.

''You can't disprove the fact that there was debris,'' she said.

She insisted, as she has before, that the cause of the crash is unimportant.

''Whether it was debris or heat, what is important is those things were forseeable when you place a helicopter that close to explosions,'' Mrs. D'Agostino said.