Tesla now has another fatality to hang on its semi-autonomous driving system. The company just revealed that its Autopilot feature was turned on when a Model X SUV slammed into a concrete highway lane divider and burst into flames on the morning of Friday, March 23. The driver, Wei Huang, died shortly afterwards at the hospital.

This is the second confirmed fatal crash on US roads in which Tesla’s Autopilot system was controlling the car. It raises now familiar questions about this novel and imperfect system, which could make driving easier and safer, but relies on constant human supervision.

In a blog post published this evening, Tesla says the logs in the car’s computer show Autopilot was on, with the adaptive cruise control distance set to the minimum. The car stays in its lane and a fixed distance from the vehicle ahead, but the driver is supposed to keep his hands on the wheel and monitor the road, too. Take your hands off the wheel for too long, and you get a visual warning, on the dashboard. Ignore that, and the system will get your attention with a beep. If you’re stubborn or incapacitated, the car will turn on its flashers and slow to a stop.

Based on data pulled from the wrecked car, Tesla says Huang should have had about five seconds, and 150 meters of unobstructed view of the concrete barrier, before the crash. Huang’s hands were not detected on the wheel for six seconds prior to the impact. Earlier in the drive, he had been given multiple visual warnings and one audible warning to put his hands back on the wheel.

The car’s manual reminds Tesla drivers that Autopilot is a driver assistance tool, not a replacement, and that they retain responsibility for driving safely. (The big center screen conveys the same message when you engage Autopilot for the first time.) But critics say the ease with which Tesla’s system handles regular freeway driving can lull a driver into thinking it’s more capable than it is, and allow them to become distracted or take their eyes off the road.

Drivers need to be ready to grab the wheel if the lane markings disappear, or lanes split, which may have been a contributing factor in this crash. Systems like Autopilot have known weaknesses. The manual also warns that it may not see stationary objects, a shortcoming highlighted when a Tesla slammed into a stopped firetruck near Los Angeles in January. The systems are designed to discard radar data about things that aren’t moving, to prevent false alarms for every overhead gantry or street-side trash can.