(Bloomberg) -- US auto safety regulators posed fresh questions this week to roughly a dozen automakers and parts suppliers in a long-running probe of air-bag inflators that could explode in a crash.

The National Highway Traffic Safety Administration sent out letters this week seeking information from the companies about the number of vehicles produced with potentially faulty parts made by ARC Automotive Inc.

The agency is also seeking information about how the companies evaluated whether the air bags would deploy normally in a crash, without a potentially dangerous rupture that could harm passengers, according to the documents posted on NHTSA’s website.

The letters highlight how air bags at risk of exploding in a crash continue to haunt the auto industry even after recalling tens of millions of vehicles with defective parts, made by the now-defunct Takata Corp., that were linked to several deaths. 

Separately, NHTSA began investigating air bag inflators made by ARC Automotive in 2015, covering millions of additional components made as far back as 2004. The most recent letters in that probe are focused on parts made by ARC between 2010 to 2018. The letters were reported earlier Thursday by the Wall Street Journal. 

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