DETROIT >> The U.S. government is now urging owners of nearly 8 million cars and trucks to have the air bags repaired because of potential danger to drivers and passengers. But the effort is being complicated by confusing information and a malfunctioning website.

The government’s auto safety agency says that inflator mechanisms in the air bags can rupture, causing metal fragments to fly out when the bags are deployed. The inflators are made by Japanese parts supplier Takata Corp.

Safety advocates say at least four people have died from the problem, which they claim could affect more than 20 million cars nationwide. On Wednesday, the National Highway Traffic Safety Administration added 3.1 million vehicles to an initial warning covering 4.7 million cars and SUVs.

Car owners might have difficulty determining if their vehicle is equipped with the potentially dangerous air bags. The warning covers certain models made by BMW, Chrysler, Ford, General Motors, Mazda, Honda, Mitsubishi, Nissan, Subaru and Toyota.

Most of the 7.8 million vehicles are subject to existing recalls. But manufacturers have limited the recalls to high-humidity areas, excluding cars and trucks in states to the north. NHTSA says owners in Florida, Puerto Rico, Guam, Saipan, American Samoa, the U.S. Virgin Islands, Hawaii and “limited areas near the Gulf of Mexico in Texas, Alabama, Mississippi, Georgia, and Louisiana” should pay special attention to the warning.

Worse yet, the regulatory agency has twice corrected the number of vehicles affected and acknowledged that a list it released Monday wasn’t completely accurate. The agency urged people to use its website to see if their cars are affected — but a feature allowing people to check for recalls by vehicle identification number malfunctioned Monday night and still wasn’t operational Wednesday.

Automakers have been recalling cars to fix the problem for several years, but neither Takata nor NHTSA have identified a firm cause. The agency opened a formal investigation into the problem in June, and a theory put forth in agency documents suggests the chemical used to inflate the air bag can be altered by high humidity, making it explode with too much force while deploying.

“It’s in a total state of uproar right now,” said Clarence Ditlow, executive director of the Center for Auto Safety, a nonprofit advocacy group founded by Ralph Nader.

The problem also is drawing attention from Congress. Staff members for the House Energy and Commerce Committee have asked NHTSA to brief them on the Takata air bags. They also plan to meet with automakers, a committee spokeswoman said.

NHTSA Deputy Administrator David Friedman said in a statement that car owners should respond to the recalls to stay safe. The agency, he said, is tracking down the “full geographic scope” of the issue.

Kathryn Henry, a spokeswoman for the agency, said it is unclear whether a high number of inquiries caused its website to malfunction. Until it’s repaired, she urged car owners to go to manufacturer websites or call dealers.

General Motors, which sold two models with the faulty air bags, planned to notify about 10,000 customers by overnight mail. The models covered are 2003 to 2005 Pontiac Vibes in high humidity areas and Saab 9-2X models. The cars were made by other manufacturers — the Vibes by Toyota, and the Saabs by Subaru.

The rare warning by regulators comes three weeks after a Sept. 29 crash near Orlando, Florida, that claimed the life Hien Thi Tran, who suffered severe neck wounds that investigators said could have been caused by metal fragments flying out of the air bag on her 2001 Honda Accord. Her Accord was among the models being recalled.

One police agency concluded that the air bags caused her wounds, while another is still investigating. NHTSA is seeking information.

On Monday, Toyota issued a recall covering passenger air bags in 247,000 older model vehicles including the Lexus SC, Corolla, Matrix, Sequoia and Tundra. Like many earlier recalls, Toyota’s recall covers vehicles only in areas that have high absolute humidity. GM and Toyota each told customers not to let anyone sit in the front passenger seat until repairs are made.

Toyota said it’s working with Takata to pinpoint the cause of the rupture and to gauge the influence of high absolute humidity, which is a measurement of water vapor in the air.