The California Highway Patrol said a catastrophic failure of the rear suspension system caused friction that ignited carpets and set the limo ablaze.
No charges will be filed in the incident, which killed five passengers. The limousine operator will be fined $1,500 for having nine passengers, one more person than is legally allowed.
A nurse and her friends were celebrating her recent wedding. Nine passengers and the driver were in the 1999 Lincoln Town Car when it caught fire on the San Mateo-Hayward Bridge on May 4.
Five passengers died in the fire. Four others and the driver survived.
Investigators reviewed video and photos and interviewed the survivors. One of the survivors had earlier said the driver was on his phone and did not help anyone escape. Authorities checked phone records and determined the driver was not on the phone at the time.
The Associated Press contributed to this report.