Federal investigators are focusing on an electric toilet-pump motor as a possible source of the intense fire that killed 23 of 46 people on board an Air Canada jetliner here Thursday en route from Dallas-Fort Worth to Toronto.

National Transportation Safety Board member Donald Engen said today the captain's log shows that three circuit breakers connected to the pump motor and other electrical equipment tripped at about the time the crew discovered smoke pouring from the left rear lavatory.

It is unknown whether those short circuits were a cause or an effect of the fire, he said.

However, he said, it seems "less likely" that a cigarette or a match tossed into a lavatory trash bin was the primary cause. The trash bin and toilet-pump motor are close to each other, Engen said.

Discovery of the smoke set off a race between pilot Donald Cameron, trying to get his stricken ship to an airport, and the smoldering, toxic fire creeping forward from the rear.

The combination of smoke in the cockpit and a sequence of electrical failures made it impossible for the pilot to see his instruments, Eengen said, so Cameron received specific turn-by-turn instructions from an air traffic controller.

Senior investigators who have heard recordings of Cameron's conversation with the controller describe it as highly dramatic and professional.

"Turn right," the controller told Cameron, for instance, then watched his radar until the proper direction had been reached and gave the pilot a new command.

The flight reached the ground 13 minutes after Cameron radioed an emergency. At the time he radioed, the plane was six miles high and 25 miles southwest of the Greater Cincinnati Airport in suburban Boone County, Ky.

The controller, based at Indianapolis, "did a fantastic job of clearing everything out of the way," Air Canada spokesman Ted Morris said.

Investigators said the controller held Cameron on one radio frequency all the way to the ground, instead of requiring him to go through the usual radio "handoffs" as the plane moved from one air-traffic control sector to another.

The pilot was so busy, they said, that he did not have time to answer important questions that rescue personnel always ask in these situations: "How many souls on board?" and "How much fuel are you carrying?"

After the plane landed and Cameron fled the cockpit, his trousers ablaze, he went to a nearby fire truck and radioed those answers to the tower, Engen said.

He said Cameron "let the plane down while the cockpit was filling with smoke. He was very calm and professional."

And the unidentified traffic controller "was superb, in my personal view," he added.

Transcripts of the pilot-controller conversation will be released in a few days, officials said.

Boone County coroner Don Stith said late today that all of the victims died of smoke inhalation, although the precise chemical nature of the smoke will not be known for another week. Stith said most of the deaths probably occurred after the landing, although some victims could have been unconscious as the plane touched down.

Most of the bodies were found in the forward section of the single-class cabin. Dr. William Gates, head of the medical team that responded to the scene, told The Cincinnati Enquirer that "there were people in the aisle stretched out, one person with a foot edged under a seat. There were people sitting in seats in a protective position, shielding faces and hands. Some were on their knees, down between the seats."

Investigators continued to study the remains of the McDonnell Douglas DC9, which is parked in a hangar. The cargo bay and luggage compartment were checked today, Engen said, and FBI agents have reached the "preliminary conclusion" that there was no "evidence of sabotage or foul play."

Investigators checking the history of this specific plane have turned up a curious but probably insignificant coincidence: it is the same Air Canada DC9 that made news in September, 1979, when its tail cone fell off over the Atlantic Ocean because of a crack in the rear bulkhead of the plane.

That flight returned without incident to Boston, and its problem led to a worldwide check of DC9s. Structural improvement of the bulkhead areas was ordered.

Investigators here were also checking the fuel system to see if a leak could have fed the fire. The aircraft's tanks were not punctured, and the plane landed carrying a substantial amount of fuel.

A small fire extinguisher was installed in the lavatory trash bin of the plane in 1982 when Air Canada completely refitted the interiors of its DC9 fleet. That unit triggered automatically at some point, Engen said, but was not sufficient to douse the fire.

The refitting included new overhead storage areas, garment closets and carpets, and seats and side panels, including panels in the lavatory. Those panels, Air Canada spokesman Patrick Daley said, are made of polyvinyl chloride and are covered with a special fire-retardant coating of polyvinyl fluoride. Those materials meet all required fire standards, he said.

This accident has rekindled a long-running debate about the safety of cabin interiors, particularly as to how much smoke they produce, their flammability and toxicity.