DETROIT (AP) ― At least seven automakers are teaming up to find the cause of a problem with air bags used in their vehicles and how many cars to recall because of it.

Inflators in certain air bags made by the Japanese supplier Takata Corp. can explode with too much force and spew shrapnel into the passenger compartment.

So far, 14 million vehicles worldwide have been recalled due to the problem, including 8 million in the U.S. Takata has yet to pinpoint a cause, even though the recalls started a decade ago. The U.S. government wants Takata and automakers to add millions of cars across the U.S. to recalls currently limited to areas with high humidity. The automakers indicated Tuesday that they want to do their own testing, in addition to tests underway at Takata.

Toyota and Honda are leading the call for an industrywide investigation. In a statement, Toyota said it will ask the industry to hire an independent engineering company, and the affected companies would share results to figure out recall repairs. So far, General Motors, Nissan, Subaru, Chrysler and Ford have agreed to cooperate. One auto industry analyst suggested the automakers feel Takata is taking too long to find the cause.

The announcement came as Takata edged closer to a midnight deadline to agree to a national recall of driver’s air bags or face civil fines and legal action from the National Highway Traffic Safety Administration. The U.S. Transportation Department, which oversees NHTSA, said Tuesday evening that the agency received a response from Takata and was evaluating it. Details weren’t available.

Some of the biggest recalls so far have been limited to high-humidity areas in the Southern U.S., plus Hawaii and some territories. The agency has said that prolonged exposure to airborne moisture can cause the inflator propellant to burn faster than designed. That can rupture metal inflator canisters. At least five deaths worldwide have been blamed on the problem.

NHTSA demanded the national recall of driver’s air bags after receiving reports of two incidents that occurred outside the recall zone. The demand covers vehicles made by Ford, Honda, Chrysler, Mazda and BMW, generally from the 2008 model year and earlier.

The government says it doesn’t have data to warrant a national recall of passenger side air bags.

In a statement Tuesday, Takata said it had formed a panel to investigate its inflator manufacturing process, but it didn’t specify whether it would agree to the national recall. Takata also said it’s working with top scientists who specialize in propellants, inflators, and air bag systems to evaluate its inflators.

Takata also appeared to pass the decision about a broader recall to automakers. The company said it would “produce additional replacement units to support any further recalls that may be announced by our customers.’’

A company spokesman said he expects a response to NHTSA later Tuesday.

On Wednesday, Takata and some of the automakers will appear at a U.S. House subcommittee hearing on the matter.

Karl Brauer, senior analyst for Kelley Blue Book, said automakers likely are fed up with how long it’s taking Takata to find the cause. But they also want their own probe so it looks like they’re protecting customers, he said.

Takata’s driver and passenger inflators are similar and use the same propellant, so Brauer wonders why all the problem air bags aren’t being recalled nationally. Regulators, he said, may be worried about Takata’s ability to handle such a large recall financially. There are more than 30 million Takata air bags in the U.S. and 100 million worldwide.

If Takata doesn’t agree to NHTSA’s demands, it could face the wrath of an agency that’s been criticized for moving too slowly on safety issues, Brauer said. “They can’t appear to be shirking that responsibility yet again,” he said.