Physical evidence from the wreckage of Northwest Airlines Flight 255 shows that the aircraft's wing flaps were set incorrectly when it departed from Detroit's Metropolitan Airport last week, federal investigators said yesterday.

The physical evidence confirms preliminary information safety officials took from the cockpit's data and voice recorders. The recorders suggested that the pilots forgot the routine procedure of turning the flaps down to help lift the jet into the air.

The plane climbed only 48 feet before slamming into a car rental agency and over a highway at the edge of the airport, killing at least 156 people in the nation's second worst aviation disaster.

When they first examined the aircraft's flight data recorder, which tracks the plane's mechanical performance, and discovered that the flaps had not been set for departure, officials said they did not believe the recorders because the maneuver is so fundamental.

But further examination convinced investigators from the National Transportation Safety Board that both recorders worked perfectly, and that the flaps and slats had been moved properly on the short trip from Saginaw, Mich., to Detroit, where Flight 255 made its final stop.

"They went down for takeoff and up on landing just like they are supposed to {on the Saginaw-Detroit flight}," said one official involved with the crash investigation. "We know the flaps were not where they should have been now. What we need to determine is where the slats were and why these things could have been out of place."

Slats are located on the leading edge of the wing and flaps on the trailing edge. Together they help deflect air over the wing during takeoff, adding to the craft's ability to leave the ground. The cockpit voice recorder recovered from the wreckage shows that the pilots failed to call off flap positions when they went through their routine pre-flight check list.

Safety board officials said yesterday in a statement that they are still uncertain whether pilot errors or mechanical problems prevented an alarm from warning the crew of the incorrect flap setting before the plane left the ground.

The aircraft, a McDonnell Douglas MD80, was equipped with a computer alarm system that should have warned the pilots that the flaps were not set for takeoff. But the warning never came, according to cockpit recordings.

Sources involved in the investigation said yesterday that, although it was too early to be sure, the circuit breakers for the warning system appeared to be in their proper place during the flight. The officials discounted earlier reports that the flight crew had disconnected a circuit breaker on the alarm system.

Safety board officials, who have concluded the initial part of their investigation into the cause of the crash, must now concentrate on why the warning system failed to work.

In its statement yesterday, the safety board said careful sound examinations will make it possible to say with certainty whether the circuit breakers on the alarm system had been pulled, either intentionally or by accident.

It is possible that a mechanical failure fooled the flight crew into assuming the flaps had descended when they had not. But, because they worked well in the previous stage of the flight, from Saginaw, that seems unlikely, officials said yesterday.

Late last week a Northwest pilot who was preparing for takeoff after Flight 255 said he was certain that the flaps were in their proper position when the plane crashed.

NTSB sources discounted that report yesterday, saying that close examination of the flaps has shown they were retracted while the plane was taking off.