Federal investigators weighed possibilities ranging from careless smoking to a fuel leak in an attempt to learn what started the fire that brought an Air Canada jetliner to an emergency landing here Thursday and killed 23 of the 46 people on board.

"There is no way of knowing" the source of the fire at this time, National Transportation Safety Board member Donald Engen said. Further, the interior of the DC9 was gutted in the intense fire that followed the landing, making the detective job more difficult.

The in-flight fire renewed a 20-year-old cry in Washington for stiff federal smoke and fire standards for materials used inside airplanes.

Once Air Canada Flight 797 landed and the plane's doors were popped open, it was "like lighting a fuse on a firecracker," according to passenger Roy Grubbs, 52, of Waxahachie, Tex. "The people who got out, got out very quickly," he said. "If you didn't get out quickly, you didn't get out."

Pilot Don Cameron of Montreal apparently escaped through a cockpit window. His pants were on fire, but the fire was quickly extinguished, according to witnesses.

The Air Canada flight, en route from Dallas-Fort Worth to Toronto, was cruising at 33,000 feet when smoke began pouring from under the door of a rear washroom, survivors told federal investigators. A flight attendant and a copilot checked the fire, and immediately radioed an emergency.

The plane was on the ground at Greater Cincinnati Airport in suburban Kentucky in 13 minutes.

Safety board investigators said today they have no idea what caused the fire. Early speculation centered, logically, on a cigarette in a trash container in the lavatory. Air Canada, like U.S. airlines, placards the lavatory areas with No Smoking signs.

One passenger suggested an electrical source; another speculated fuel. Investigators were also checking the possibility that the fire began in the cargo area. Whatever the source, once the fire got started, it spread forward through the plane with ever-increasing speed.

An in-flight fire is a rarity, but the problem of the rapid spread of flame, smoke and toxic fumes has been well-known since the 1960s, when autopsies of many crash victims showed that they had survived the crash only to die of asphyxiation or poisoning in the post-crash fire.

Almost everything on an airplane--including the aluminum skin--will burn if it gets hot enough. The wall and ceiling panels, the carpet, the seats and cushions are manufactured from lightweight plastics. When ignited, those materials emit fumes that include large quantities of carbon monoxide and hydrogen cyanide, both deadly.

It was against that background of common knowledge that pilot Cameron and the copilot, who also survived but has not been identified, began their rapid descent into Cincinnati.

Surviving passenger Graham Wright, 42, of Toronto, said there was "absolutely no doubt in my mind" that if the flight had lasted "one minute more none of us would have made it."

The safety board's Engen said the doors and overwing exits were opened and the escape slides and chutes deployed immediately after the plane landed. The captain applied maximum braking power and blew all the tires. Fire trucks stood at the ready.

The passengers, meanwhile, had moved forward in the cabin as the smoke and fumes advanced from the rear. Randy Morris, of Denton, Tex., said that "by the time we landed, the smoke was so thick and it was so hard to breathe and you couldn't see a thing."

"I never saw any flames until I opened the hatch," Morris said. "Flames leaped out after us." He said that before the plane landed, flight attendants asked everyone to move forward. "We felt it was safer to stay next to an overwing exit," he said. "So we did."

Morris said there was no panic and no hollering. "In fact, the plane was real quiet."

Jack Barry, assistant director of airport operations here, said that within 30 seconds of the landing, "everybody who was able to get off the plane had gotten off. The real blaze was after the plane landed."

Engen said most of the fatalities appeared to be in the forward and mid-cabin areas.

One of the rescue workers, a look of horror in his eyes, described on local television "the melted bodies" of those unable to escape.

The plane has been taken to a hangar here for inspection. Its interior is gutted; large holes were burned in the top of the fuselage.

Safety board officials were hoping that interviews with the five crew members will help them develop a clearer picture of what happened. Eighteen passengers also survived. A total of 18 people were hospitalized.

The cockpit voice recorder was taken to Washington for analysis; the flight data recorder, which will tell about aircraft and engine performance, was taken to Canada.

As long as the plane is airborne, the oxygen supply is somewhat suppressed, so a fire tends to smolder and smoke. Once the plane's doors are opened and oxygen enters, intense fire builds rapidly. Some survivors here told the safety board of a "fireball" that roared through the cabin after the exits were opened.

Federal regulations require only that fabrics and materials used in airplanes pass a flammability test; there are no rules on smoke and toxicity. Members of Congress, especially Reps. Norman Y. Mineta (D-Calif.) and Elliott H. Levitas (D-Ga.), have pushed the Federal Aviation Administration for years to strengthen the standards.

During the Carter administration the FAA withdrew all proposed rules on smoke and toxics for more study. The technical problem, experts have said, is that changing chemical plastic formulas to reduce flame will frequently increase smoke or toxicity, and vice versa. In 1979 House panel hearings, Eugene Bara of Boeing was asked by Mineta if cabin materials "with improved fire safety characteristics should be an important part of our overall air safety effort."

Bara replied: "It is obviously an important part, but the total solution to the hazard will not occur simply by working the interior materials. You will get an increment of improvement. I think the increment is very finite and very small."