Sound Transit says Friday's frigid temperatures and a frozen brake line could be to blame for a bus fire on Interstate 5 in north Seattle that blocked the express lanes through the morning rush hours.

Spokeswoman Kimberly Reason says the cause of the fire is under investigation but it could have been caused by frozen brakes that overheated.

The driver detected something wrong, pulled over and safely evacuated about 70 passengers, including a woman in a wheelchair.

Reason says the driver works for a Snohomish County Community Transit contractor.

Flames spread through the diesel-powered articulated bus and it burned for more than half an hour as Seattle firefighters poured on water and foam.

The Transportation Department says the fire at North 85th Street backed up traffic for 10 miles.