DENVER -- Investigators with the National Transportation Safety Board are looking for the cause of a bumping and rattling sound heard on a Continental Airlines plane that veered off a runway and crashed at Denver International Airport Saturday evening.

An initial review of the voice and data recorders also shows the crew noticed the plane drifting from the center line as it sped down the runway, and tried to stop the takeoff.

Investigators returned to the plane's wreckage at the airport on Tuesday, rushing to collect evidence ahead of snow expected to move in later in the day, National Transportation Safety Board spokesman Peter Knudson said. One group was working inside the charred fuselage to document its condition while experts continued to analyze information from the plane's recorders.

The odd noise was first heard 41 seconds after the plane started down the runway, said Robert Sumwalt, a spokesman for the NTSB.

Four seconds later, one of the crew members called for the takeoff to be aborted. The recording ends six seconds after that, probably because the plane slammed down on the ground after hurtling off an embankment, Sumwalt said.

All passengers and crew members escaped the plane, which caught fire on the right side. Thirty-eight people were injured, including the plane's captain.

The conditions of the four people still hospitalized Tuesday were the same: two were listed in good condition, one in fair and one in serious condition. The captain was among the four.

Sumwalt said investigators have found no problems with the plane's engines, tires or brakes, but are not yet ruling anything out.

The plane traveled about 2,000 feet after leaving the runway, crossing a grassy strip and a taxiway before going off the embankment, hitting the ground at its base. It then went up a slight hill, over an access road and then down another small hill on the other side of the road before landing on its belly, its landing gear shorn off.

Lead NTSB investigator Bill English said the plane's data recorder shows the thrusters on both engines were switched to reverse. He said that normally happens when crew members try to stop a takeoff.

Sumwalt said investigators are still gathering information about the exact wind conditions on the runway at the time of the accident. However, he said the cockpit recording contained no comments about wind.

Investigators have not yet interviewed the plane's captain, who was flying the plane, because Sumwalt said he is physically unable. He didn't elaborate. They have talked to the first officer, who said the plane began moving off the center of the runway as it reached about 103 mph while speeding down the runway for takeoff.

The plane continued to accelerate, reaching a maximum speed of about 137 mph, Sumwalt said. Take-off speed for a 737 is about 150 mph or 130 knots.

Off-duty crew members who had flown the plane earlier in the day also were on board at the time of the accident, and Sumwalt said the first officer from that crew returned to the plane three times to help rescue passengers. Sumwalt also reported that those crew members said they had no problems with the plane during their flight.

A fire charred and ripped open much of the right side of the plane, with the worst damage around a crack around the fuselage. Sumwalt said all the passenger seats remained intact during the plane's wild ride off the runway, although seats in row 18, near the crack, had loose fittings.

Sumwalt said the runway was bare and dry at the time of the crash and no debris was found there.

DIA spokesman Jeff Green said all airport's runways were open Tuesday.