WASHINGTON, Aug 11 (Reuters) - Two U.S. lawmakers who chair subcommittees overseeing auto safety asked the federal auto safety regulator for a briefing on its probes into crashes involving Tesla Inc (TSLA.O)electric vehicles using Autopilot and advanced driver assistance systems, according to a letter seen by Reuters.<p>U.S. Senator Gary Peters and Representative Jan Schakowsky, both Democrats, said in the letter to the National Highway Traffic Safety Administration (NHTSA) they were concerned that "federal investigations and recent reporting have uncovered troubling safety issues" at Tesla.<p>The lawmakers asked "given the mounting number of fatalities involving Tesla vehicles crashing into tractor trailers ... has NHTSA considered opening a defect investigation into this issue?"<p>The letter added "does NHTSA strike a balance between investigative thoroughness and addressing urgent, emerging risks to motor vehicle safety?" and if the agency has enough resources to properly investigate advanced driver assistance systems.<p>Tesla did not immediately comment.<p>Since 2016, NHTSA has opened 38 special investigations of crashes involving Tesla vehicles and where advanced driver assistance systems such as Autopilot were suspected of being used. A total of 19 crash deaths have been reported in those Tesla-related investigations.<p>Last month, NHTSA said it opened a special investigation into the crash of a 2020 Tesla Model 3 vehicle that killed a motorcyclist in Utah.<p>In June, NHTSA upgraded its defect investigation into 830,000 Tesla vehicles with Autopilot involving crashes into parked emergency vehicles, a required step before it could seek a recall.