Two high-profile limousine accidents in Northern California this spring are raising questions about oversight of the industry that builds the vehicles — though highway safety advocates see little prospect of tougher scrutiny by lawmakers anytime soon.

“The idea of Congress getting involved at this point might be a bit of a stretch,” said Henry Jasny, vice president and general counsel of the group Advocates for Highway and Auto Safety. “If there’s a pattern of safety issues, we’d see folks at [the National Highway Traffic Safety Administration] getting involved. Right now, it’s not clear that there’s a single issue.”

The pair of California accidents that have focused attention on limousine safety appear to be unrelated.

In the first incident, a stretch limousine carrying a bridal party across the Bay Bridge connecting San Francisco and Oakland caught fire, killing five and injuring four others. Preliminary indications from an investigation suggest a failed suspension part contributed to the problem. In another incident, a group of elderly women were forced to flee a stretch limousine when they noticed smoke. All got out alive.

The Federal Motor Carrier Safety Administration and NHTSA have a variety of regulations in place for the manufacturing and operation of vehicles designed to carry more than nine passengers. But smaller limos — generally ordinary luxury vehicles that are converted by small operations — are subject to less regulation.

“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,” Joan Claybrook, who headed NHTSA during the Carter administration and led the consumer advocacy group Public Citizen for many years, told The Associated Press last month. “Mostly, they are mom-and-pop operations.”

NHTSA does keep records on accidents and actively polices recalls on limousines, just as it does with regular passenger vehicles.

From 2001 to 2003, for example, the agency conducted an in-depth investigation into fuel tank fires with some Lincoln Town Car models popular with limousine companies after reports of fires following rear-end crashes.

Ford Motor Co. responded to the investigation in 2005 by offering more than 32,000 fire-protection kits for cars it manufactured, but Clarence Ditlow, executive director for the Washington-based Center for Auto Safety, criticized the company for not recalling the vehicles.

That same type of limousine was involved in the fatal fire on the California bridge, but investigators have said the blaze didn’t involve a rear-end crash. Moreover, because of the way limousines are built, such an investigation won’t be as cut and dry as one involving regular passenger vehicles.

Generally, a secondary manufacturer takes ordinary large sedans or sport utility vehicles, slices them in half, then lengthens them and adds luxurious features inside the vehicles to produce limousines.

“The second-stage manufacturer is just as liable under the law,” said Jasny, the safety attorney and advocate. “But that doesn’t mean it’s inherently safe.”

Ditlow said that two-step building process raises a lot of questions that would be best answered by investigative experts at the National Transportation Safety Board.

As an independent government advisory, the NTSB retains top field experts to investigate crashes and infrastructure failures of all sorts but also faces severe resource constraints.

“It’s unfortunate because they have the knowledge and know-how and the capacity to influence policy decisions,” Ditlow said. “But the industry is so small in comparison, it just doesn’t get the notice.”

The Federal Motor Carrier Safety Administration has been cracking down on high-risk limousine operators as part of “Operation Quick Strike,” an enforcement drive implemented this year. The effort stems from a series of fatal bus accidents in recent years, including a 2011 crash in the Bronx.

The agency’s website also provides a searchable database with safety records of carriers licensed to operate across state lines.

Industry officials say they are committed to working with investigators and regulators to ensure the safety of limousines.

“We can say with confidence that this tragic accident was an anomaly for our industry,” Philip S. Jagiela, executive director of the National Limousine Association, said about the fatal California accident. “Once a cause is determined, we will work … to chart a constructive approach and course of action for our members.”