LOS ANGELES, FEB. 7 -- The air traffic controller handling two planes that collided on a runway at Los Angeles International Airport, killing 34 people thought one of the planes was on a taxiway and that the runway was clear, a federal investigator said tonight.

The unidentified controller was interviewed for three hours Wednesday by investigators from the National Transportation Safety Board who sought to learn what happened in a 10-minute period Friday night before the collision between a USAir Boeing 737 and a smaller SkyWest commuter plane.

Investigators have described the heavy traffic load the controller was juggling just before the collision. She was directing four planes.

The controller told the NTSB that she directed the SkyWest pilot to enter runway 24 L-6R and to hold in position for takeoff, investigator James Burnett said at a news briefing this evening. Seconds later, she cleared the USAir jet to land on the same runway, he said.

The Boeing 737 smashed the smaller plane as it landed, skidded down the runway in a raging fireball and slammed to a halt into an unused building. Survivors have described a mad scramble for exits through heavy smoke.

The controller explained that before the crash, she looked at the runway and saw a plane she thought was the SkyWest commuter sitting blocked from the runway behind a waiting Southwest Airlines 737, Burnett said.

Even after the crash, she was unsure what caused it, he said. "She saw an explosion," Burnett said. "She believed a bomb had exploded."

All 12 people on the SkyWest plane were killed. Twenty-two aboard the USAir jet, which carried 89 passengers and crew, died in the crash. Four of 67 crash survivors remained hospitalized. Some victims apparently died of smoke inhalation, several apparently fighting with each other over who would leave the plane first. Some bodies were found in front of a door that was apparently stuck or blocked shut.

Investigators also have said a ground radar display, which might have reminded the controller that the SkyWest plane already was on the runway, was broken.

The Los Angeles County coroner's office identified four more victims of the crash who were on the USAir jet. They are Rosemary Weth, 59, of McLean, Va.; Dawn Withers, 24, of Gahanna, Ohio; Jimmy Perdue, 19, a Navy sailor from Huntsville, Ala.; and David Sharp, 44, of Cheltenham, England.