U.S. safety regulators on Tuesday said that about 19 million defective Takata Corp (7312.T) air bag inflators remain installed in U.S. vehicles, and that no root cause to the problem has been found. About 19.2 million vehicles containing a total of 23.4 million air bag inflators made by 11 automakers have been recalled, a NHTSA official said. The inflator figure is higher because about 4 million vehicles have defective Takata air bag inflators on both driver and passenger sides, the National Highway Traffic Safety Administration official said.