DETROIT—A second person has been killed by an exploding air bag inflator made by a Tennessee company that has been under investigation by a federal agency for more than six years without any resolution. On Wednesday, the National Highway Traffic Safety Administration posted recall documents filed by General Motors that revealed the second death, the driver of a 2015 Chevrolet Traverse SUV with an ARC inflator that blew apart, spewing shrapnel. No details were given about where and when the death occurred. NHTSA has said that ARC Automotive of Knoxville has manufactured about 8 million inflators used nationwide in vehicles made by General Motors, Fiat Chrysler (now Stellantis), Kia and Hyundai. Some auto safety advocates say the investigation has dragged on for too long, and is an example of deadly consequences of an understaffed and underfunded agency. “NHTSA should have been all over this along time ago,” said Rosemary Shahan, …