The U.S. government’s auto safety agency, responding to criticism of its slow response to safety issues, told the manufacturer of millions of potentially faulty air bags to make replacement parts faster and do more testing to find the cause of the problem.

The National Highway Traffic Safety Administration sent letters Wednesday to Japanese air bag maker Takata Corp. and 10 automakers seeking information in a widening air bag recall that now covers almost 8 million U.S. vehicles.

The vehicles are equipped with Takata air bags that can potentially inflate with too much force, blowing apart metal canisters and sending shards flying at drivers and passengers. Safety advocates say four people have died due to the problem.

Tests by Takata have shown that prolonged exposure to high humidity can cause the inflators to malfunction. Some automakers have limited their recalls to a small number of high-humidity areas, but lawmakers and others are demanding that recalls be expanded nationwide.

Takata, the world’s second-largest air bag maker with 22 percent of the market, has been plagued by problems for the past 13 years. For varying reasons, more than 12 million cars with its air bags have been recalled worldwide.

Honda has been hit hardest in the latest round of recalls with about 5 million cars called back. Other affected automakers include Nissan, Chrysler, Ford, Mazda, Subaru, Mitsubishi, Toyota, BMW and General Motors. Lawmakers say that 30 million cars with potentially faulty Takata air bags are driving on U.S. roads.

In the letter, NHTSA Deputy Administrator David Friedman told Takata that its inflators are “creating an unacceptable risk of deaths and injuries by projecting metal fragments into vehicle occupants rather than properly inflating the attached air bag.”

The letters sent to automakers urge them to speed up owner notification and replacement part distribution. Friedman told Takata that those efforts won’t work if it doesn’t produce enough parts.

“Takata’s production capacity is critically important,” Friedman wrote.

The letter states that Takata has agreed to take “aggressive steps” to accelerate production, but Friedman demands that the company state its current production capacity and the ability to expand it and how long it would take. He also asks about the possibility of getting replacement parts from other inflator manufacturers, and how the company is tracking inventory and quality verification.

“Production of replacement parts must not just be expedited, it must be prioritized,” Friedman wrote.

He also wrote that Takata has promised to double its testing program for inflators returned from recalled cars.

A message was left Wednesday seeking comment from Takata.

NHTSA also asked the automakers for test results on air bags that were sold outside high-humidity areas. Automakers have different boundaries for recall zones, which has confused consumers. NHTSA says the affected area includes Florida, Puerto Rico; limited areas near the Gulf of Mexico in Texas, Alabama, Mississippi, Georgia, and Louisiana; as well as Guam, Saipan, American Samoa, Virgin Islands and Hawaii.

Former Hyundai North American CEO John Krafcik, who now heads the TrueCar.com auto pricing site, said a nationwide recall is the only way to solve the problem.

“I think you still have that obligation to ensure that there’s not another death, another lost eye, another sliced jugular vein,” he said, in reference to some of the reported injuries.

NHTSA has said it has no data showing the problem has happened outside high humidity areas.

Also Wednesday, Democrats and Republicans on the House Energy and Commerce Committee asked the nonpartisan Government Accountability Office to investigate NHTSA. The committee contends the agency responds slowly to safety issues.

(AP)