Federal safety regulators are asking vehicle manufacturers about potentially defective air-bag inflators estimated to be in tens of millions of cars, following a string of recall campaigns and accidents. 

In letters sent this week to about a dozen auto makers and suppliers, the National Highway Traffic Safety Administration said it was seeking information on inflators used for both passenger and driver-side air bags that were made by Knoxville, Tenn.-based ARC Automotive Inc. 

The letters mark the latest step in an investigation begun more than seven years ago by NHTSA, the nation’s top auto-safety regulator, into ARC’s inflators. The components have exploded at least six times in vehicles on U.S. roads in crashes that have left two people dead and four injured, according to NHTSA filings and court documents.

ARC didn’t immediately respond to a request for comment. NHTSA declined to comment about the letters. 

So far, General Motors Co. , Ford Motor Co. , BMW AG and Volkswagen AG have issued seven relatively small recall campaigns linked to NHTSA’s examination covering about 6,400 vehicles with ARC inflators. The most recent came in July, when VW said it was recalling about 1,200 cars in response to an inflator rupture in a 2016 Audi A3 e-tron that injured the driver. 

Those auto makers were among the companies that received letters from NHTSA this week. VW, Ford and BMW didn’t immediately respond to requests for comment. 

GM said it has been working with NHTSA on its investigation and has retained an independent third-party engineering firm with expertise in air-bag inflator performance. The auto maker said it now has no evidence of a systemic manufacturing or design defect in ARC-built inflators and it isn’t aware of evidence that would support expanding prior recalls to cover more vehicles.

NHTSA has said the probe covers an estimated 51 million inflators, but has declined to cite the number of vehicles potentially affected. The ARC inflators were installed in cars made between model years 2002 and 2017, records show. 

Plaintiffs’ attorneys who have filed pending lawsuits against ARC and car makers estimate that more than 30 million cars have air-bag inflators covered by the probe.

The scope of the vehicle population affected by the ARC investigation is similar to the Takata Corp. air-bag recall campaign, which began in 2014 and covers roughly 42 million vehicles, the largest in U.S. history. Last month, NHTSA warned 276,000 drivers of vehicles with still unrepaired Takata air bags to get their cars fixed, following recent fatal crashes. 

In the letters sent this week regarding the ARC inflators, NHTSA asked auto makers to provide details on the number of vehicles that contain the allegedly defective parts and provide assessments of the probability that future abnormal deployments won’t occur. 

The request covers vehicles produced from mid-2014 through January 2018, the end date reflecting when ARC made equipment and process improvements to its assembly lines, according to the letters. 

The regulator began its investigation into ARC’s inflators in July 2015. At the time, the agency said it was prompted by two incidents involving ARC inflators that ruptured and injured drivers. 

The agency expanded its examination the following year after learning of an additional incident involving a Canadian woman who was killed by air-bag shrapnel that exploded in a crash. Since last year, additional cases have triggered the more recent recalls launched by auto makers.

Write to Ryan Felton at ryan.felton@wsj.com