DETROIT (AP) — Federal safety regulators are investigating a problem with side air bags that may fail to inflate in a crash.

The problem already has caused recalls of more than 2,700 Toyota, Honda, Subaru and Nissan vehicles, but that number could grow if the National Highway Traffic Safety Administration determines that more automakers used similar defective parts.

The recalled vehicles could have an ineffective mixture of the gas that inflates the side curtain air bags in a crash. That mix could mean air bags on one or both sides of the cars won't inflate in a crash, increasing the risk of injury, the agency said in documents posted on its website. So far no one has been hurt.

All four companies with recalled vehicles told NHTSA that the Americas unit of Swedish safety parts maker Autoliv Inc. made the propellant for the side air bags. The investigation was opened Feb. 14. Documents detailing the probe were posted on NHTSA's website last weekend.

"At this time it is unknown what other equipment manufacturers or vehicle manufacturers received the affected Autoliv propellant mixture," NHTSA said in documents detailing the probe. The agency said it would write Autoliv to find out which companies got the same mixture. The agency said more recalls could be coming.

Autoliv confirmed on Tuesday said the problem occurred one time in production parts tests at minus-22 degrees Fahrenheit. But further tests indicated a risk of malfunctions at low temperatures, and customers who used the parts were told of the problem, spokesman Mats Odman wrote in an email.

"We are in a dialogue with our customers and the authorities to ensure that they are fully informed and that the driving public could rely on our products," Odman wrote.

He did not answer a question about what other automakers are using the same inflators.

Autoliv also supplies parts to General Motors Co., Ford Motor Co., Volkswagen AG, Hyundai Motor Co. and Kia Motors Corp., according to its website, but it was unclear whether those companies got the same air bag inflators.

Toyota Motor Corp. and Honda Motor Co. first discovered the problem and reported it to NHTSA on Jan. 31, the documents said.

The recalled vehicles include 427 Toyota RAV-4s from the 2011 model year that were made from Nov. 24, 2011 through Dec. 19, 2011. Also included are 974 Honda Accords, Civics, Crosstours and Acura MDXs from the 2012 model year made between Nov. 30, 2011 to Jan. 26, 2012; 381 2012 Subaru Legacy and Outback cars made from Dec. 1, 2011 through Dec. 23, 2011; and 976 Nissan Altimas and Versa cars from the 2012 model year that were made from Nov. 21, 2011 to Dec. 21, 2011.