Federal investigators said yesterday that the pilot of a Pacific Southwest Airlines jetliner that crashed in the nation's worst air disaster Monday may not have seen the light plane that struck the jet's wing because he was watching a second plane that had invaded the same airspace.

Philip A. Hogue, director of the National Transportation Safety Board's investigation of the crash, said a study of the tapes of conversations between the jetliner pilots and air traffic controllers indicated that a second plane, a twin-engine Cessna, was spotted by the pilots directly ahead of the jetliner.

But it was another plane, a single-engine Cessna 172, which collided with the jetliner, sending it plunging to earth. At least 150 persons - 135 passengers and crew, two in the light plane, and at least 13 on the ground - were killed in the crash.

Hogue said the pilot of the PSA Boeing 727 was warned that he was very close to a light plane and reported to air traffic controllers that he saw a twin-engine Cessna.

"The pilot of the jet said, 'They've passed' but we're unsure which plane he was referring to," Hogue said.

He said the twin-engine plane was trying to get clearance from the Lindbergh Field control tower to land on the same runway as the jetliner. He said it was too early to tell if the unidentified twin-engine plane had caused the collision. He said the plane had apparently landed safely and added that investigators were trying to identify it.

Hogue said the pilot of the jetliner showed no fear after the collision. "The PSA pilot said, 'We're going down,' and he said it in the softest of tones," Hogue said. "There was no sign of panic, no hint of terror."

The final number of dead caused by the crash is still not known. Coroner's deputies were sifting through the rubble yesterday in the residential neighborhood in central San Diego searching for the bodies of people still reported missing by their families.

"They wouldn't have had to live here. They just could have been walking their dog around the block or driving through," a Red Cross spokesman said.

The fact that the jetliner went down in a residential neighborhood has rekindled a long-standing debate in this southern California coastal city of 1 million about where the airport should be located.

Four years ago, Lindbergh Field, this city's downtown airport next to the harbor, was termed "an accident waiting to happen" by an airline pilot. The pilot urged that a new airport be constructed in an outlying area away from all development, such as Dulles Airport in the Washington area.

San Diego Mayor Pete Wilson, who has long battled for a new airport, was restrained on the issue yesterday when visiting the devastated site of the crash, but mentioned the dangers of having jet flight paths over residential areas. Wilson and other city officials noted that a Catholic school and a Navy hospital were near the crash site, and said that the disaster could have been much more.

The pilot of the single-engine Cessna 172 was practicing landings and was intending to land on the same runway that PSA Flight 182 was heading for when the two planes collided. The Cessna pilot, who was being trained, wore a mask that restricted his vision to the instrument panel, a common method of teaching pilots to land at night or in bad weather.

Federal Aviation Administration officials said yesterday that, "conditions permitting," major airports can allow practice instrument landings, but a survey of several indicates such runs are usually forbidden.

Washington National Airport "discourages practice landings" but they are not forbidden. Chicago's O'Hare, Dallas-Fort Worth, Boston's Logan, San Francisco and New York's Kennedy prohibit them.

San Diego's Lindbergh Field is the only airport in the immediate area with the capability to practice instrument approaches, however.