REDWOOD CITY, Calif. — A mechanical problem is to blame for igniting a limousine fire that killed five nurses who were trapped in the back, the California Highway Patrol said Monday.

CHP Captain Mike Maskarich says the blaze broke out on the San Mateo-Hayward Bridge on May 4 because of a catastrophic failure of the rear suspension system. The suspension fell onto the floor pan causing friction that ignited carpets and set the vehicle on fire, authorities said.

Authorities say no charges will be filed. The Public Utilities Commission is fining the limo operator $1,500 for having more passengers than allowed.

The fire broke out while a nurse, Neriza Fojas, was celebrating her recent wedding with a group of friends.

She was among the five killed. Four other friends inside the limo and the limo driver survived.

Authorities reviewed video and photos of the fire and interviewed survivors, including the limo driver, Orville Brown.

Brown, 46, of San Jose, said at first he misunderstood what one of the passengers in the back of the 1999 Lincoln Town Car was saying when she knocked on the partition window.

With the music turned up, Brown said he initially thought the woman was asking if she could smoke. Seconds later, he said, the women knocked again, this time screaming, “Smoke, smoke!” and “Pull over.”

Brown said he helped the four survivors escape through the partition. One of women ran around to a rear passenger door but by then the vehicle was engulfed in flames.

One survivor, a sobbing Nelia Arellano, told KGO-TV a few days after the fire that Brown “didn’t do anything” to help the women escape the car. In a May 7 interview, Arellano told NBC Bay Area that Brown was on the phone.

Brown’s brother, Lewis Brown, an attorney based in Vallejo, denied the accusations to NBC Bay Area.

Authorities said they reviewed Brown’s telephone records and that he was not on the phone during the accident.

The state Public Utilities Commission had authorized the vehicle to carry eight or fewer passengers, but it had nine on the night of the fire.

Aerial video shot after the incident showed about a third of the back half of the limousine scorched by the fire. Its taillights and bumper were gone and it appeared to be resting on its rims, but the remainder of the vehicle didn’t appear to be damaged.