SAN FRANCISCO — Investigators are trying to determine why the back of a stretch limousine burst into flames on a San Francisco Bay bridge, trapping and killing five of the nine women inside on a girls’ night out, including a newlywed bride.

It happened late Saturday night as the Lincoln Town Car crossed the San Mateo-Hayward Bridge on the south end of the bay.

The driver, Orville Brown, said at first he misunderstood what one of the passengers in the back was saying when she knocked on the partition between the passenger area and the driver compartment and complained about smelling smoke.

With the music up, 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 told the San Francisco Chronicle.

He helped four of the surviving women escape through the partition. One of the women ran around to the passenger door on the back side of the limo, but by then it was engulfed in flames.

“When she opened that back door, I knew it wasn’t a good scene,” Brown said. “I figured with all that fire that they were gone, man. There were just so many flames. Within maybe 90 seconds, the car was fully engulfed.”

Two of the women who survived were in critical condition on Monday.

The five women trapped inside were found dead as firefighters doused the vehicle — all huddled near the partition, apparently unable to squeeze through.

“My guess would be they were trying to get away from the fire and use that window opening as an escape route,” said San Mateo County Coroner Robert Foucrault.

The California Highway Patrol scheduled a news conference with fire officials for 10 a.m. Monday. The San Mateo Fire Department was looking into the cause of the fire, while the coroner’s office was working with the California Highway Patrol to determine if anything criminal occurred.

“We don’t believe there” was, Foucrault said.

Relatives told the San Francisco Chronicle and the San Jose Mercury News that one of the dead was Neriza Fojas, 31, a registered nurse from Fresno who recently wed and was planning to travel to her native Philippines to hold another ceremony before family. Her friends in the limousine were fellow nurses.

Brown, 46, of San Jose, said he was taking the women across the bridge to the Crowne Plaza Hotel in Foster City. Fojas’ sister, Rosalyn Bersamin, told the Chronicle that after a night out on the town, Fojas and her friends were heading to the hotel to party with her new husband.

“She was a hard worker, a loving sister,” a sobbing Bersamin said.

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

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

Brown said he wishes he could have done more.

“It’s something you never imagine will happen,” he told the Chronicle. “It’s a limousine ride. It’s supposed to be a joyous thing.”

Brown said he is an experienced commercial driver who has operated airport shuttles and bobcat trucks.

He started driving a limousine for the limo company, Limo Stop Inc., two months earlier and had put in about six shifts behind the wheel of the Town Car that caught fire, he said.

Medical examiners will identify the victims by using dental records. Foucrault said the autopsies will include toxicology tests, as well as examinations into whether any accelerant such as alcohol or gasoline was found on the bodies.

The CHP said the four other women who escaped the fire were being treated at nearby hospitals for burns and smoke inhalation: Mary G. Guardiano, 42, of Alameda; Jasmine Desguia, 34, of San Jose; Nelia Arrellano, 36, of Oakland; and Amalia Loyola, 48, of San Leandro.

Desguia and Loyola were listed in critical condition Monday, a spokeswoman for Valley Medical Center said. The condition of Arrellano, who was taken to another hospital, was not known.

A spokeswoman for Community Medical Center in Fresno said one or more of its employees were in the limo.

Limo Stop offers service through limousines, vans and SUVS.

The company issued a statement saying it “will do everything possible to investigate and assist authorities in determining the cause of this fire in order to bring forth answers and provide closure to [the] victims and their families.”

According to records from the California Public Utilities Commission, which regulates limousine companies, Limo Stop is licensed and insured.