People inspect the Tesla Model 3 as it sits on display at the Los Angeles Auto Show last December.

The driver killed while operating a Tesla on Autopilot in March may not have his hands on the steering wheel just before the crash, the National Transportation Safety Board said in a preliminary report Thursday.

The report indicates what happened in the minutes and seconds before a 38-year-old man slammed a Tesla Model X into the crash cushion at the end of a freeway median and collided with two other vehicles in Mountain View, California. Up to the time of the accident, the driver was operating the car using Tesla's advanced driver assistance system, Autopilot.

The system has been investigated by federal agencies for previous crashes.

The vehicle warned the driver with two visual alerts and one sound alert telling him to place his hands on the wheel more than 15 minutes prior to the crash, the NTSB said. The driver placed his hands on the wheel three times in the minute before the accident, for a total of 34 seconds. In the final six seconds, the system did not sense he had his hands at the wheel.

At six seconds prior to the crash, the car sped up from 62 miles per hour to 70.8 mph, the agency said.

A Tesla spokesperson referred CNBC back to Tesla's blog on the accident, and declined to comment further.

"The reason this crash was so severe is because the crash attenuator, a highway safety barrier which is designed to reduce the impact into a concrete lane divider, had been crushed in a prior accident without being replaced," the blog said. "We have never seen this level of damage to a Model X in any other crash."

CORRECTION: This story was revised to correct that the NTSB's preliminary report said Tesla's Autopilot did not detect driver's hands on the steering wheel just before the fatal crash. A previous version misstated the timing.