Peritz said the fire was believed to have started in the bus's brake system and may have caused oxygen canisters on the bus to explode. The number of dead from the incident could get higher.

Authorities blocked all lanes of the interstate, complicating the already grueling exodus from the Texas coast.

The bus was carrying 38 residents and six employees of the Brighton Gardens nursing home in Bellaire, according to Sunrise Senior Living, the McLean, Va., company that owns the center.

The National Transportation Safety Board dispatched an investigation team from its office in Austin. But Peritz said early indications were that the bus, which had been on the road since Thursday, caught fire because of mechanical problems, possibly overheated brakes. Then passengers' oxygen tanks started exploding, turning the vehicle into an inferno, he said.

The fire reduced the vehicle to a blackened, burned-out shell with large blue tarps covering bodies, surrounded by about 20 police cars and ambulances.

At least nine people were injured.

Sunrise said it was contacting family members of the victims.