Astronauts Judy Resnick and Ellison Onizukawere riding just behind Smith and Scobee, and theymay have assisted Smith, he said.

"The most plausible explanation is that one ofthe two reached over and turned on Mike's airpack," Truly said.

Kerwin said that it was possible the crewmembers lost consciousness due to a loss ofpressure inside the crew compartment. The loss ofpressure could be deadly, even if the crew memberswere breathing from the air packs, he said.

The packs, designed to be used for a shuttleemergency on the ground, normally hold about fiveminutes worth of air, NASA said.

Investigators say the accident occurred when ajoint in a solid rocket booster failed and causedsuperheated gases to burn through the externalfuel tank containing liquid hydrogen and liquidoxygen, triggering the explosion.

The crew cabin tore loose at 45,000 feet, arcedupward to about 65,000 feet, and then began a2-minute, 45-second plunge to the Atlantic Ocean,Kerwin said.

Acceleration forces at the time of the breakupwere estimated at 12 to 20 times the force ofgravity for about 2 seconds, the report said.

"Medical analysis indicates that theseaccelerations are survivable, and that theprobability of major injury to crew members islow," it said.

An analysis showed that if the crew memberslost consciousness due to a loss of pressure, theywould not have had time to revive as the crewcompartment fell into denser air at loweraltitudes, Kerwin said