Federal investigators Friday turned to the first officer of Delta Flight 1141 to find out what caused the jetliner to stall and crash while trying to take off.

The team from the National Transportation Safety Board, which began its investigation within hours of the crash at 9:03 a.m. Wednesday, focused early on the wreckage but Thursday and Friday turned to interviews with the crew and witnesses in the airplane and on the ground.Investigators were talking Friday to the first officer, Wilson Kirkland Jr., 37, of Monroe, La., said a spokesman at NTSB's command post at Dallas-Fort Worth International Airport.

The crash killed 13 of the 108 people aboard. Thirty-three people remained in hospitals, with one of them in critical condition and three others in serious condition.

An interview with the pilot, Larry Davis, 48, of Greenville, Texas, will be delayed because of his broken bones and internal injuries. He was upgraded from serious to fair condition but was still considered in too much pain to be subjected to an in-depth interview.

Lee Dickinson, leader of the NTSB team, said Thursday evening other pilots who saw the crash, either from their airplanes or the DFW control tower, said Flight 1141's left engine began burning and trailing smoke during the overly long takeoff run. The jetliner used the entire 11,000 feet of the runway, rather than normal 6,800 required for a 727-200 to take off, and only achieved a maximum of 30 feet of altitude before it stalled and crashed.

Dickinson Thursday night said investigators found the cockpit flap indicator in an "up" position and that the plane's flaps "were not fully extended." But he cautioned that the control may have been hit by the flight crew when it evacuated the plane. Wing flaps in the up position would prevent the craft from achieving enough lift for takeoff.

Ninety-five people survived the fiery crash, and Delta officials Thursday said one reason so many escaped may be that a dozen off-duty Delta employees helped keep order as passengers crawled through emergency exits and holes in the plane's damaged fuselage.

NTSB officials in Washington made a preliminary review Thursday morning of the plane's voice recorder, Dickinson said, and it indicated that the flight crew received warning of a possible stall and discussed the possibility of engine trouble before the crash.

"From a one-time-only listening shortly after takeoff there was a sound from the `stick shaker,' which gives warning that a stall condition may be approaching," he said. "Then there was a conversation about some engine problem."

In aviation terminology, a "stall" occurs when there is insufficient air moving over the airplane's wings to lift the aircraft, a Delta spokesman said.

Dickinson said transcripts of the voice recorder would be released within 60 days.

Some passengers praised Capt. Davis for selflessness in verbally guiding them as they walked to safety past him. "You almost had to step on the debris that was on the pilot (to get out)," passenger David Carmichael said. "And the whole time he was saying, `Slow down, watch your step.' "

Besides the cause of the crash, NTSB officials are interested in learning how so many people were able to escape the burning aircraft. Many of those who fled the plane were uninjured until they leaped to the ground or were burned on the plane's scalding fuselage.