National Transportation Safety Board (NTSB) investigators say an unusual rattling sound can be heard on the cockpit voice recorder of the Continental Boeing 737-500 shortly before it veered off a runway after an aborted takeoff from Denver International on Dec. 20.

Lead NTSB investigator Bill English said the jetliner’s flight data recorder showed that the thrust-reversers on both engines were activated, according to the Associated Press.

NTSB Member Robert Sumwalt said the jet began drifting off the center of the runway as it reached about 103 mph, then veered off the runway, sliding across a field and into a ravine.

NTSB officials are trying to identify the unusual sound, and whether any engine or brake malfunctions may have contributed to the accident. Sumwalt said the rattling sound was first heard 41 seconds after the plane started down the runway, according to the Associated Press. Four seconds later, one of the crew members called for the takeoff to be aborted.

Sumwalt said investigators have found no problems with the plane's engines, tires or brakes, but aren't ruling anything out. The aircraft’s flight control surfaces -- including flaps, slats and speed brakes – appeared to be in proper position.

Sumwalt revealed the findings late Monday after an initial review of the flight data and cockpit voice recorders. A more in-depth analysis of the contents of the ‘black boxes’ is underway.

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 voice recorder contained no comments about excessive crosswinds or windshears.