A federal investigation said yesterday it "appears remote" that a third plane confused the pilot of a jetliner seconds before he collided with a single-engine Cessna, resulting in the death of at least 150 persons.

It remained a mystery what the pilot meant when he told the tower, "They've passed," the investigator said.

The third plane, a twin-engine Cessna, actually was eight miles west of the area where the Pacific Southwest Airlines Boeing 727 and a single-engine Cessna 172 collided on approach to Lindbergh Field Monday morning, said Phillip Hogue, an investigator for the National Transportation Safety Board.

All 135 persons aboard te jet, both persons in the small plane and 13 persons on the ground were killed, making it the worst air disaster in U.S. history.

"Whether it (the twin-engine Cessna: would be a factor seems remote at his time," Hogue said. "It would seem to eliminate the third aircraft, but I can't eliminate it totally."

The probable elimination of the third plane theory "still leaves the questions of who or what he (PSA pilot James McFeron) saw," Hogue said. "We've got pieces of the puzzle. We're trying to put them together."