(CNN) -- The fire that engulfed a limousine on a bridge over the San Francisco Bay in May -- killing five members of a bachelorette party, including the bride-to-be -- was the result of friction between the car's drive shaft and the rear floor board, investigators said Monday.
They ruled the fire an accident and said no criminal charges will be filed.
Nine passengers were traveling in the limousine as it crossed the San Mateo-Hayward Bridge on night of May 4, including 31-year-old Neriza Fojas, who was celebrating her upcoming wedding.
Driver Ricky Brown told CNN two days later that as he drove over the bridge, one of the passengers knocked on the partition separating him from the back and said "smoke."
He thought she was asking to smoke a cigarette, he said, so he told her to wait just a few minutes because smoking wasn't allowed inside the vehicle.
After 30 seconds, she knocked again, Brown said. That's when he smelled smoke and pulled over.
Cell phone video shot by another driver shows the limo at the side of the bridge, the back engulfed in flames and the front in smoke.
A "catastrophic failure" of the limousine's rear suspension system allowed the rapidly spinning drive shaft to come into contact with the floor pan, Capt. Mike Maskarich of the California Highway Patrol's Redwood City office said.
As the fire grew, it ignited the back seat, creating black smoke and flames that filled the passenger compartment, Foster City Fire Chief Michael Keefe said. The smoke and fire blocked access to the rear doors, leaving passengers with only one exit -- the small opening into the driver's compartment, he said.
San Mateo County District Attorney Steve Wagstaffe said Monday his office looked for anything that could have constituted a crime but found nothing.
"We've explored all possibilities, and the conclusion is that this is a horrific tragedy," he said.
Whatever went wrong with the suspension system happened after the limo started crossing the bridge, investigators said. Video of the vehicle going through the bridge's toll plaza does not show it riding low to the ground.
The investigation showed, however, that the limousine was carrying two more people than allowed. It was rated for seven passengers but was carrying nine.
The owner of the limousine company will be fined $7,500 for failing to operate safely, because there weren't enough seat belts for all nine passengers, said Brig. Gen. Jack Hagan, director of the safety and enforcement division of the California Public Utilities Commission, which oversees passenger carriers like limousines.