Then the taps turned to urgent knocks, and someone screamed "Smoke, smoke" and "Pull over!"
In just a few fleeting moments, five of the women celebrating a girls' night out were killed by flames that overtook the luxury car with terrifying speed.
As smoke thickened in the passenger compartment, Brown pulled the white stretch limo to a stop on a bridge over San Francisco Bay and started pulling women out through the partition that separated him from his passengers.
Three good Samaritans, including a firefighter, stopped to help. The first woman who got out ran to the back and yanked open a door, but Brown said it was already too late.
"I knew it wasn't a good scene. I figured with all that fire that they were gone, man," Brown said. "There were just so many flames. Within maybe 90 seconds, the car was fully engulfed."
From the first tap on the window until the rear of car became an inferno couldn't have taken more than three minutes, Brown told the San Francisco Chronicle.
Authorities searched for answers Monday, hoping to learn what sparked the blaze and why five of the victims could not escape the fast-spreading flames.
The women who were killed in the Saturday night blaze were found pressed up against the 3-foot by 1 ½-foot partition, apparently because smoke and fire kept them from the rear exits of the extended passenger compartment.
The position of the bodies suggested they were trying to get away from the fire, said San Mateo County Coroner Robert Foucrault.
The women were celebrating the wedding of a newlywed friend, Neriza Fojas, who was among the dead.
Fojas and another of the fatalities, Michelle Estrera, were nurses at Community Regional Medical Center in Fresno. The remaining three victims have not been identified.
The medical center's CEO, Jack Chubb, said in a statement Monday that Fojas and Estrera were outstanding nurses, loved by their patients, colleagues and staff.
"Both were good friends, stellar nurses and excellent mentors who served as preceptors to new nurses," he said. "We'll dearly miss these two special people who have touched our lives."
A relative of Fojas said the young nurse was preparing to get her master's degree and was planning a large second wedding in the Philippines.
Christina Kitts said Monday that Fojas lived in Hawaii while she reviewed for her nursing exam, then took a job in Oakland for two years before moving to Fresno, where she had been a nurse at Community Regional Medical Center for a year.
Three survivors hospitalized were identified as Jasmine Desguia, 34, of San Jose; Mary Guardiano, 42, of Alameda; and Amalia Loyola, 48, of San Leandro.
Nelia Arellano, 36, of Oakland, who was treated and released, told KGO-TV about the terrifying events.
With a cut visible on her face, an emotional Arellano said she yelled at the driver to stop the car, but he "didn't want to listen."
When Brown did finally stop, Arellano says he did nothing to help the women get out of the burning car after he exited.
Brown told KGO that at first he misunderstood what one of the passengers was saying when she knocked on the partition and talked about smoke. And when the panicked woman knocked a second time and yelled at him to stop, he said he pulled over and all four survivors escaped through the partition. But he said the passenger compartment was quickly engulfed in flames.
"It spread so fast," he said.
Brown said he believed it was an electrical fire. "It could have been smoldering for days," he said, noting there was no explosive boom.
California Highway Patrol Commander Mike Maskarich said the state Public Utilities Commission had authorized the vehicle to carry eight or fewer passengers, but it had nine on the night of the deadly fire. Maskarich said it was too early in the investigation to say whether overcrowding may have been a factor in the deaths.
Commission spokesman Terrie Prosper said Monday that the agency was looking into whether the operator of the limo, a company called Limo Stop, willfully misrepresented the seating capacity to the agency. If so, Limo Stop could be penalized $7,500 for each day it was in violation.
Limo Stop is licensed and has shown evidence of liability insurance, Prosper said. The company has seven vehicles with a seating capacity of up to eight passengers listed with the commission, and it hasn't been the target of any previous enforcement action.
The CPUC requires that all carriers have a preventive maintenance program and maintain a daily vehicle inspection report, Prosper said. Carriers also certify that they are have or are enrolled in a safety education and training program, she said.
Prosper said requirements for emergency exits only apply to buses, and limousines are not required to have fire extinguishers.
Joan Claybrook, the top federal auto-safety regulator under President Jimmy Carter, said the stretch limousine industry is poorly regulated because the main agency that oversees car safety doesn't have enough money to prioritize investigating the small businesses that modify limos after they leave the assembly line.
"I think the oversight is pretty lousy, because the modifications are so individualistic, and there are not that many companies out there that do this. Mostly, they are mom-and-pop operations," said Claybrook, a former administrator at the National Highway Traffic Safety Administration who previously led consumer group Public Citizen.
Instead, the agency tends to focus more on problems with new cars and major recalls, she said.
U.S. Department of Transportation data shows five people died in three separate stretch limo accidents in 2010, and 21 people died in another three stretch limo accidents in 2011.
Stretch limos are typically built in two ways.
In the first process, one carmaker builds the limousine's body, then another company customizes or stretches the vehicle. The second company has to issue a certification that the car meets National Highway Traffic Safety Administration safety standards for new vehicles, and that all safety equipment is working as required before it can be sold to the public, said Henry Jasny, an attorney with the Washington-based nonprofit Advocates for Highway and Auto Safety.
In the second process, a customer buys the limousine directly from the carmaker, then takes it to be customized. But modifying the car after it has been sold is considered a retrofit, so is not something NHTSA would regulate, Jasny said.
Many older models such as the 1999 Lincoln Town Car that caught fire Saturday were modified after they left the factory, said Jerry Jacobs, who owns a boutique limousine company in in San Rafael with a fleet that includes two stretch limos.
"There is nothing wrong with having these older models on the road. Many have low mileage and immaculate interiors because we take care of them. But when these cars start getting older and the rubber boots wear out, they start running hot," Jacobs said. "The key is you have to keep doing all the right maintenance to make sure they're running smoothly."
Associated Press writer Sudhin Thanawala in San Francisco contributed to this report.