The National Transportation Safety Board is investigating a fatal crash involving a Tesla Model X that occurred last Friday morning in Mountain View, California. The agency is looking into whether Tesla’s semi-autonomous Autopilot feature had anything to do with the crash and is investigating a fire that resulted from the car’s battery system. The severity of the accident is unprecedented, according to Tesla. “We have never seen this level of damage to a Model X in any other crash,” the company wrote in a blog post on Tuesday. 

The driver of the car, Wei Huang, was headed southbound on California’s Route 101 when his Model X crashed headfirst into the safety barrier section of a divider that separates the carpool lane from the off-ramp to the left. The front end of his SUV was ripped apart, the vehicle caught fire, and two other cars crashed into the rear end. Huang was removed from the vehicle by rescuers and brought to Stanford Hospital, where he died from injuries sustained in the crash, according to Mercury News.

The batteries that power Tesla’s vehicles are specifically designed to prevent “thermal runaway,” which is when heat building up in the battery pack causes chemical reactions that, in turn, cause even more heat. “Tesla battery packs are designed so that in the rare circumstance a fire occurs, it spreads slowly so that occupants have plenty of time to get out of the car,” the company writes. “According to witnesses, that appears to be what happened here as we understand there were no occupants still in the Model X by the time the fire could have presented a risk.”

It’s unclear how long the fire lasted or how accurately Tesla’s second-hand account describes what happened. A spokesperson for the NTSB said in an email that the “field investigation, focused on the post crash fire and steps necessary to safely remove and transport the vehicle from the scene, continues.”

Tesla says it believes this crash was so serious because the safety barrier, which is supposed to mitigate the amount of impact with the concrete divider behind it, was either damaged or had been reduced in size. The company obtained images taken a day before the crash from the dash cam footage of a person who claims to have witnessed the accident. It shows that the barrier was a fraction of the size that it appears in older Google Street View photos. In 2017, The Model X became the first SUV to get a five-star safety rating across the board from the National Highway Traffic Safety Administration.

Tesla also claims that its owners have driven by this same particular barrier while using Autopilot “roughly 85,000 times since Autopilot was first rolled out in 2015” without any accidents. (Tesla states in its privacy policy that it collects data remotely about the use of Autopilot, among other things.) The NTSB said earlier this week that it is “[u]nclear if automated control system was active at time of crash.” Tesla says it is “working closely” with investigators to recover data from the vehicle, which could help explain what happened, and that it proactively reached out to the NTSB. 

The NTSB previously investigated Tesla’s Autopilot feature after a driver died from a collision with a tractor-trailer in 2016. The agency found that Autopilot operated mostly as intended, but it “gave far more leeway to the driver to divert his attention to something other than driving,” which contributed to the crash. The NTSB has also recently looked into a January 2018 accident where the driver of a Model S claims to have been using Autopilot when the car crashed into a fire truck.

Semi-autonomous driving systems are currently experiencing enhanced scrutiny after a vehicle in Uber’s test fleet of autonomous cars killed a pedestrian in Arizona earlier this month. A number of companies, most notably Toyota and Nvidia, have suspended testing efforts in response.