DETROIT--U.S. auto safety regulators said Monday they are monitoring data from a group of mostly unrecalled Takata air bag inflators after one of them exploded in a BMW and hurled metal fragments that seriously injured a driver in Chicago.

The National Highway Traffic Safety Administration has been investigating the batch of about 30 million inflators since 2021 to see if they exhibit the same traits that forced Takata to recall 67 million of the devices since 2001.

On Saturday, the agency posted documents showing that BMW is recalling 486 SUVs after the Chicago driver was hurt. A complaint filed with the agency shows that on Oct. 23, the inflator on a 2014 X3 exploded, shooting a large gold-colored metal disc that a surgeon had to remove from the driver’s lung.

Takata used volatile ammonium nitrate to create a small explosion to inflate air bags in a crash. But the chemical propellant can deteriorate over time when exposed to high temperatures and humidity. It can explode with too much force, blowing apart a metal canister and spewing shrapnel.

At least 26 people have been killed in the U.S. by Takata inflators since May 2009, and at least 30 have died worldwide including people in Malaysia and Australia. In addition, about 400 people have been injured. The exploding air bags sent Takata of Japan into bankruptcy.

Separate from the recalled Takata inflators are about 30 million that contain a moisture-absorbing chemical called a desiccant that is meant to keep the ammonium nitrate dry and stable.

The potential for a dangerous malfunction led to the largest series of auto recalls in U.S. history. About 100 million inflators were recalled worldwide.

In a statement, BMW blamed the ruptured inflator on a welding defect in manufacturing and said it is limited to a small lot of inflators. The automaker said the problem is “in contrast to previous recalls related to Takata airbag gas generators in which the aging of the propellant was the issue.”

The BMW recall, however, is the third from the group of inflators with a desiccant, raising questions about their safety.

General Motors recalled nearly 900 vehicles in July with inflators that have the desiccant and also blamed the problem on a manufacturing defect at Takata. In 2020, Volkswagen recalled about 370,000 Passat and Beetle cars from 2012 through at least 2016 due to a propellant problem. The phased recall was to run through early 2025.

In a statement Monday, NHTSA said the GM and BMW recalls came from the same group of inflators “and appear to stem from manufacturing issues rather than propellant degradation.” The VW inflators had a different propellant mixture than was used in the GM and BMW vehicles, it said.

“NHTSA continues to closely monitor all air bag inflator issues and will take action if a safety defect is identified,” the agency said in a statement.

But in a document that opened the 2021 investigation, NHTSA said it understands that desiccants will become saturated at some point and will not capture any more moisture. This means the deterioration process that happens in inflators without the desiccant may also happen in inflators with the desiccant, the document said.

Michael Brooks, executive director of the Center for Auto Safety, a nonprofit advocacy group, questioned whether the recalls are the start of problems with the inflators that have the desiccant.

Automakers, he said, blamed manufacturing problems to limit recalls when the non-desiccated Takata inflators started to have problems. “It’s just the usual ‘trust us. Oh, it’s going to be fine. It’s not the whole fleet,” he said.

Companies want to save money by delaying any recalls, Brooks said. “The longer you delay any recall, the more of those vehicles are going to go out of service.”

BMW said in documents posted Saturday that the recall covers 486 X3, X4 and X5 SUVs from the 2014 model year. In the BMW recall, dealers will replace the air bags at no cost to owners, who will be notified by letter starting Jan. 16.

The investigation opened in 2021 covers more than 30 million inflators in over 200 models from 20 car and truck makers, including Honda, Stellantis, General Motors, Ford, Nissan, Tesla, BMW, Toyota, Jaguar Land Rover, Daimler Vans, Mitsubishi, Subaru, Mercedes-Benz, Ferrari, McLaren, Porsche, Mazda, Karma, Fisker, Spartan Fire vehicles.