Cars are parked in the employee parking lot at Tesla Inc's U.S. vehicle factory in Fremont, California, U.S., March 18, 2020. REUTERS/Shannon Stapleton/File Photo

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.

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.

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?"

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 and legal authority to properly investigate advanced driver assistance systems.

NHTSA did not immediately comment. In July, NHTSA Administrator Steve Cliff told Reuters he wanted to complete the investigation into Tesla's advanced driver assistance system Autopilot "as quickly as we possibly can but I also want to get it right. There's a lot of information that we need to comb through."

Tesla, which has disbanded its press office, did not immediately comment. Tesla's website says that Autopilot enables vehicles to steer, accelerate and brake automatically and "require active driver supervision and do not make the vehicle autonomous."

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.

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.

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. That investigation was first launched in August 2021.

On June 15, NHTSA said Tesla reported 273 vehicle crashes since July 2021 involving advanced driving assistance systems, more than any other automaker.

The lawmakers asked if NHTSA has determined whether Tesla has implemented safeguards to prevent advanced driver assistance systems "from being activated when the vehicle is not within the appropriate operating conditions?"

The National Transportation Safety Board and others have questioned if Tesla is doing enough to ensure drivers pay attention while using Autopilot.

"Has any NHTSA investigation found that a design which allows the operation of an ADAS in circumstances it is not designed for constitutes a defect?" the lawmakers wrote, using the acronym for advanced driver assistance systems.

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Audi of America, Kia Corp and Porsche said on Friday that buyers of its electric vehicles will lose access to federal tax credits of up to $7,500 once President Joe Biden signs a $430 billion climate, health and tax measure.

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