SILVER SPRING, Md.—The U.S. government’s highway safety regulator has opened an investigation into a potential problem that surfaced last year with 56 million Takata airbags but that the agency eventually deemed safe based on industry research. The National Highway Traffic Safety Administration has not posted an official document on the investigation yet, but said that it wasn’t aware of any injuries or fatalities due to the problem and that the public “does not need to take any action.” The agency said Monday its investigation will try to determine if there’s a defect in non-recalled frontal and passenger inflators manufactured by Takata, adding that “no present safety risk has been identified.” Takata had used the volatile chemical ammonium nitrate to create a small explosion to inflate air bags in a crash. But it was discovered that the chemical can deteriorate over time when exposed to high heat and humidity. It can …