Story highlights Boeing 737's nose gear collapsed on landing in New York

Plane skidded down runway before coming to a stop; passengers evacuated down slides

Several people hurt in incident involving largest domestic airline

National Transportation Safety Board investigating; Southwest

The Southwest Airlines jet that skidded down a LaGuardia airport runway on its nose on Monday touched down on its front wheels first, which then collapsed, according to federal accident investigators.

The unusual landing, in which investigators said the plane's nose pitched down seconds before touchdown, is the surest clue yet to explain the accident involving the Boeing 737 that injured several people.

The National Transportation Safety Board said on Thursday that video and other sources provide evidence that the nose gear contacted the tarmac before the main wheels.

Under a normal landing in a big jet, the main landing gear under each wing would touch the runway first and simultaneously -- absorbing the main stress of landing -- and the nose gear then lowered gradually as the plane decelerates down the runway.

That model jet is expected to cross the runway threshold at about 140 knots or 161 mph, according to Boeing figures.

Investigators said the pilot evidently adjusted the wing flaps less than a minute before landing, suggesting to veteran pilots who spoke with CNN that the plane may not have been on a stabilized approach as it neared the runway.

Landing the way it did "would potentially have overstressed the nose gear to the point where it would have failed," said Capt. Mark Weiss, a former 737 pilot and civil aviation leader at The Spectrum Group in Washington.

The NTSB said the flaps were set from 30 to 40 degrees about 56 seconds prior to touchdown.

"What this brings into question is whether they were on a stabilized approach to make a normal landing," Weiss said.

The safety board, coincidentally, is looking at whether Asiana Flight 124, which crashed-landed in San Francisco earlier this month, was also on a stabilized approach.

Details released late Thursday about the New York accident show the plane changing its pitch suddenly in the final seconds of flight -- and in a direction opposite than desired.

When the plane was just 32 feet in altitude, four seconds before landing, the plane was pitched 2 degrees nose up. At touchdown, the plane was pitched approximately 3 degrees nose down, the safety board said.

Video shows the plane careening down the runway, the nose to the pavement, sending up showers of sparks. After touchdown, the aircraft came to a stop within approximately 19 seconds, the NTSB reported.

Southwest said in a statement that the landing scenario the NTSB describes from video and other sources "is not in accordance with our operating procedures."

Word that the landing was unusual could remove suspicion of a mechanical failure. Such a conclusion could have major implications for commercial aviation worldwide, given the 737's status as the most widely used commercial jet.

The company operates only 737s and has more than 600 in its fleet.