A stretch limousine taking nine women to a bachelorette party erupted in flames, killing five of the passengers, including the bride-to-be, authorities and the mother of one of the survivors said Sunday.

The limo caught fire around 10 p.m. Saturday on one of the busiest bridges on San Francisco Bay, California Highway Patrol officer Art Montiel said.

Five of the women were trapped, but four others managed to get out after the vehicle came to a stop on the San Mateo-Hayward Bridge, the patrol said. The driver of the limousine — 46-year-old Orville Brown of San Jose — was the only person to escape unhurt.

Rosita Guardiano told the San Francisco Chronicle that the woman for whom the bachelorette party was being thrown was to be married next month. Guardiano said her daughter was one of the survivors.

Investigators haven’t determined what sparked the fire, but the patrol said the white stretch limo became engulfed in flames after smoke started coming out of the rear of the vehicle.

A photo taken by a witness and broadcast on KTVU-TV showed flames shooting from the back.

Aerial video shot after the incident showed about one-third of the back half of the limousine had been 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.

It wasn’t clear if any other drivers on the bridge stopped and tried to help the women get out, or how the driver managed to escape without injury.

The company that operated the limo was identified as Limo Stop, which operates limousines, vans and SUVs. A call left at the company seeking comment was not immediately returned.

Autopsies were being conducted to determine the exact causes of death, San Mateo County Supervising Deputy Coroner Michelle Rippy said. All five women were pronounced dead at the scene.

Attemptsto reach the driver for comment were unsuccessful.

Guardiano said her daughter — 42-year-old Mary Grace Guardiano of Alameda — was being treated for smoke inhalation.

The other three women who escaped the fire, Jasmine Desguia, 34, of San Jose; Nelia Arrellano, 36, of Oakland; and Amalia Loyola, 48, of San Leandro, were taken to area hospitals where they were being treated for smoke inhalation and burns, the patrol said.