Tesla SUV, on Autopilot, Accelerated Before Fatal Mountain View Crash

Updated 3 p.m. Thursday

Federal safety officials say that a Tesla SUV involved in a fatal crash March 23 on U.S. 101 in Mountain View was being operated in the vehicle's semi-autonomous mode and accelerated sharply just before it slammed into a highway barrier.

The National Transportation Safety Board's preliminary report on the crash gives a second-by-second account of the Model X's behavior as it approached the exit from southbound U.S. 101 to Highway 85.

The agency says data retrieved from the vehicle showed that the Tesla-branded Autopilot system was engaged for the 18 minutes and 55 seconds before the crash that killed the driver, 38-year-old Apple software engineer Walter Huang.

The report says the system cautioned Huang several times -- twice with visual alerts, once with an audio alert -- that he needed to put his hands on the steering wheel. In the final minute before the crash, Huang's hands were detected on the steering wheel three times for a total of 34 seconds. His hands were not on the wheel, the report says, for the six seconds before impact.

The report also includes this timeline of the final moments before the crash:

As the car approached the Highway 85 exit, a high-speed ramp on the left side of southbound U.S. 101, Huang's Model X was following another vehicle and traveling at 65 mph. Seven seconds before the crash, the Tesla's speed dropped slightly and it began to move toward the left -- into the exit lane -- still following the other car. Four seconds before the crash, data showed, the Autopilot system no longer detected the car ahead.

As the car entered what highway engineers call the "gore" area -- the triangular-shaped zone where the exit lane diverged from U.S. 101 -- the Model X accelerated.

Huang neither applied the brakes nor tried to steer the car, the report says, before the Tesla hit a damaged crash attenuator. That's a device installed to protect vehicles from hitting the exposed end of a concrete highway barrier.

In its damaged state -- it had been hit by another vehicle 11 days before the Tesla crash -- the attenuator could do nothing to reduce the force of the impact. In a statement March 30, Tesla cited the crash attenuator as a major factor in the severe damage the crash caused.

Asked whether the company could offer any insight into the car's behavior before the crash -- specifically its acceleration before it hurtled into the crash attenuator -- a Tesla spokesperson referred KQED to the March 30 statement and declined comment.