By David Shepardson. REUTERS

Two U.S. agencies on Monday said they were investigating a Tesla (TSLA.O) crash in Texas on Saturday that left two dead and which local police said appeared to have occurred with no one in the driver's seat.

The National Highway Traffic Safety Administration (NHTSA), which regulates vehicle safety, and the National Transportation Safety Board (NTSB) both said they would investigate the crash.

The NHTSA said it "has immediately launched a Special Crash Investigation team to investigate the crash. We are actively engaged with local law enforcement and Tesla to learn more about the details of the crash and will take appropriate steps when we have more information."

Just hours before the crash, Tesla Inc Chief Executive Elon Musk tweeted to his more than 50 million followers, "Tesla with Autopilot engaged now approaching 10 times lower chance of accident than average vehicle."

Tesla, whose shares were down 3.4% Monday, did not immediately comment.

The crash occurred as scrutiny is increasing over Tesla's semi-automated Autopilot driving system following recent crashes.

Autopilot was operating in at least three Tesla vehicles involved in fatal U.S. crashes since 2016. NHTSA has sent teams to at least three other Tesla crashes in recent weeks that were believed to have been tied to Autopilot use.

In Saturday's accident, the 2019 Tesla Model S was traveling at high speed near Houston when it failed to negotiate a curve and went off the road, crashing into a tree and bursting into flames, local television station KHOU-TV said.

After the fire was extinguished, authorities located two occupants in the vehicle, with one in the front passenger seat and the other in the back seat of the Tesla, KHOU-TV said, citing Harris County Precinct 4 Constable Mark Herman.

Tesla advises drivers to keep their hands on the steering wheel and pay attention while using Autopilot. However, some Tesla drivers say they are able to avoid putting their hands on the wheel for extended periods when using Autopilot.

Senator Richard Blumenthal, a Democrat, wrote on Twitter that "using Tesla’s driverless system — or any other — shouldn’t be a death risk. Advancements in driving technology must first & foremost be safe."

He added that "comprehensive oversight" by the NHTSA "is paramount to prevent future semi-automated driving deaths."

Last month, NHTSA told Reuters it had opened 27 special investigations into crashes of Tesla vehicles, 23 of which remain active, and that at least three of the crashes had occurred recently.

The NTSB, which makes safety recommendations but cannot compel recalls, said it would send two people to conduct a safety investigation into the Texas crash focusing "on the vehicle’s operation and the post-crash fire."

The NTSB last criticized the NHTSA’s approach to oversight of automated vehicles as "misguided, because it essentially relies on waiting for problems to occur rather than addressing safety issues proactively." It added that NHTSA has "taken a nonregulatory approach to automated vehicle safety."