The Tesla Model X that crashed in California earlier this year while being guided by its semi-autonomous driving system sped up to 71 miles an hour in the seconds before the vehicle slammed into a highway barrier, investigators said Thursday.

U.S. National Transportation Safety Board issued a preliminary report on the March 23 crash showing that the driver’s hands were detected on the steering wheel only 34 seconds during the last minute before impact.

The investigation is the latest to shine a spotlight into potential flaws in emerging autonomous driving technology. Another NTSB probe of a self-driving Uber Technologies Inc. car that killed a pedestrian March 18 in Arizona found that the car’s sensors picked up the victim, but the vehicle wasn’t programmed to brake for obstructions.

Walter Huang, a 38-year-old engineer who worked at Apple, died in Mountain View, California, in the March 23 crash when his Model X struck the barrier as he was using the driver-assistance system known as Autopilot. The car’s computer didn’t sense his hands on the steering wheel for six seconds before the collision, according to NTSB.

Tesla shares, which had been up 3.3 percent, erased much of their gains on the news. The shares were up less than 1 percent to $322.45 at 10:38 a.m. in New York trading.

The preliminary report didn’t include conclusions about what caused the crash. “All aspects of the crash remain under investigation as the NTSB determines the probable cause, with the intent of issuing safety recommendations to prevent similar crashes,” the report said.

A Tesla spokeswoman declined to comment on the NTSB’s report and pointed to a March 30 company blog post. In the post, the company said the driver had about five seconds and 150 meters of unobstructed view of the highway barrier but took no action to avoid the collision, citing vehicle logs.

“Tesla Autopilot does not prevent all accidents — such a standard would be impossible — but it makes them much less likely to occur,” the company wrote in the post. “It unequivocally makes the world safer for the vehicle occupants, pedestrians and cyclists.”

Huang was using Tesla’s Autopilot system continuously for nearly 19 minutes prior to the crash. The system made two visual and one auditory alert for the driver to place his hands on the steering wheel, but those occurred more than 15 minutes prior to the crash, according to the report.

The NTSB didn’t report any alerts in the moments leading up to the crash.

The Tesla was following a lead vehicle at about 65 miles per hour 8 seconds prior to the crash. A second later, the car began to steer left while still following the lead vehicle. Four seconds before the crash it was no longer following the lead vehicle, the NTSB said.

The Model X then accelerated from 62 mph (99 kilometers per hour) to 70.8 mph in the final three seconds before impact. The Autopilot’s cruise control system, which is designed to match the speed of a slower vehicle ahead of it, was set at 75 mph.

The Tesla collided with a so-called crash attenuator, a device covering the concrete barrier that’s designed to absorb a vehicle impact to lower risks of damage and injuries. The attenuator had been damaged 11 days earlier in a previous accident and hadn’t been repaired, according to NTSB. The barrier is in the median of the highway where it splits into two different directions.

No pre-crash braking or evasive steering movement was detected, according to NTSB’s summary of performance data recorded by the car.

While Tesla tells drivers they must keep their hands on the steering wheel and monitor the semi-autonomous system, the car can steer and control speed in some situations.

The NTSB originally announced it was looking into a fire that erupted in the car’s battery, which was damaged in the impact. The agency is also investigating a May 8 fire in a fatal Tesla crash in Fort Lauderdale, Florida.

Once the company said that the Autopilot system was switched on in the Mountain View crash, the safety board broadened the investigation to include the vehicle’s automation.

Two consumer advocacy groups charged May 23 that Tesla’s promotional material on Autopilot are deceptive. A Tesla website says its vehicles have “full self-driving hardware.” The site also contains a video of a car navigating streets without human input with text saying “the car is driving itself.”

Tensions in the NTSB probe boiled over on April 11 when Tesla released information about the accident without first clearing it with investigators, prompting the NTSB to take the unusual action of removing the car company from official participation.

Tesla had issued comments blaming the driver of the Tesla SUV. “While we understand the demand for information that parties face during an NTSB investigation, uncoordinated releases of incomplete information do not further transportation safety or serve the public interest,” NTSB Chairman Robert Sumwalt said in a statement.

Tesla Chief Executive Officer Elon Musk hung up on Sumwalt as he explained the removal, according to the NTSB chief.

Bloomberg’s Susan Decker contributed to this report.