A National Transportation Safety Board investigation into a fatal crash came down hard on both Tesla (TSLA -1.38%) and a regulatory body charged with overseeing highway safety, raising fresh questions about the electric vehicle manufacturer's Autopilot system.

The NTSB at a hearing concerning a fatal 2018 Model X crash concluded that engineer Walter Huang was driving with Autopilot engaged when his vehicle hit a highway barrier and playing a game on his phone. NTSB vice chairman Bruce Landberg called Tesla's Autosteer feature, which is designed to keep an automobile in its lane while driving on highways, "completely inadequate."

A Model Y on a course outside of an urban area.

Tesla's forthcoming Model Y. Image source: Tesla.

The hearing is sure to renew debate over Tesla's aggressive push into autonomous driving, and the safety of its software systems. Tesla's Autopilot documentation requires drivers to remain alert and focused on the road even while using Autopilot, but NTSB officials at the hearing called into question whether those guidelines are being followed.

The NTSB also had harsh words for the National Highway Traffic Safety Administration, which critics have argued has been lax in developing autonomous and semi-autonomous driver guidelines. The NHTSA in a statement said it is "aware of" the NTSB report and "will carefully review it," noting that "NHTSA has, for years, recommended that developers of advanced driver assistance systems incorporate appropriate driver-vehicle interaction strategies in deployed technology."

It's too soon to judge the longer-term ramifications of the NTSB's comments. The report seems likely to add fuel to calls by Massachusetts Senator Edward J. Markey's push to force Tesla to rebrand Autopilot, and could cost Tesla valuable credibility in its quest to win the self-driving race.

Then again, in the years since the crash Tesla has made adjustments to Autopilot, including lowering the time before a hands-off warning sounds, that it hopes will make future incidents less likely.