Updated

A bus carrying nursing home residents fleeing from Hurricane Rita (search) caught fire and was rocked by explosions Friday on a gridlocked highway near Dallas, killing as many as 24 people, authorities said. "Deputies were unable to get everyone off the bus," Dallas County Sheriff's Department spokesman Don Peritz said.

He said he believed 24 people were killed, but that number could change. The Dallas County medical examiner was still working to determine the number of fatalities, officials said.

The bus was carrying 38 residents and six employees of the Brighton Gardens (search) nursing home in Bellaire, according to Sunrise Senior Living (search), the McLean, Va., company that owns the center. The bus was en route to another home owned by the same company, according to a Baylor University Medical Center (search) spokeswoman.

Early indications were that the bus caught fire because of mechanical problems, then passengers' oxygen tanks started exploding, Peritz said. He said the brakes may have been on fire. The bus was engulfed with flames, causing a lengthy backup on Interstate 45 already congested with evacuees from the Gulf Coast.

Sunrise said it was contacting family members.

"Our primary concern is for the safety of our residents, and we are shocked and saddened that this event occurred during our evacuation," Sunrise chairman and chief executive Paul Klaasen said in a statement.

The National Transportation Safety Board (search) sent a team of specialists and investigators to the scene.

The interstate was shut down about four hours but reopened after authorities made the unusual decision to move the blackened, burned-out shell of the bus quickly so hurricane evacuees could get through.

"You have thousands of people who are in their vehicles trying to escape," Peritz said.

Tina Jones, a nurse from Ennis, was driving behind the bus when she saw it start to smoke and pull to the side of the road.

"I saw the smoke, and then there was an explosion," said Jones, who pulled over and helped treat cuts and bruises. She said she saw at least six bodies.

"I'll probably go home and have a good cry," she said.

Fred Witte, 74, said he heard three explosions from his property about 150 yards from where the bus caught fire.

"I was right there at the corner, and I felt the pressure," he said of the first explosion.

Peritz said the driver survived and helped emergency workers try to save people, but that there wasn't much time before explosions ended rescue efforts.

"It's my understanding he went back on the bus several times to try to evacuate people," he said.

No emergency workers suffered major injuries.

Dr. Paul Pepe, Parkland's chairman of emergency services, said one woman was in critical condition and being treated for smoke inhalation, but that most injuries were minor. A Parkland spokeswoman said the hospital was treating nine patients.

Baylor spokeswoman Maria Carpenter said the hospital had treated and released four patients, including two elderly sisters.

Interstate 45 stretches more than 250 miles from Galveston through Houston to Dallas. The crash site is roughly 17 miles southeast of downtown Dallas.