The National Highway Traffic Safety Administration on Thursday said it was upgrading its probe into 830,000 Tesla vehicles with its advanced driver assistance system Autopilot, a required step before it could seek a recall.

The auto safety agency in August opened a preliminary evaluation to assess the performance of the system in 765,000 vehicles after a series of crashes in which Tesla vehicles struck stopped emergency vehicles. NHTSA is upgrading its probe to an engineering analysis, which it must do before it could demand a recall if deemed necessary.

NHTSA said the upgrade is “to extend the existing crash analysis, evaluate additional data sets, perform vehicle evaluations, and to explore the degree to which Autopilot and associated Tesla systems may exacerbate human factors or behavioral safety risks by undermining the effectiveness of the driver’s supervision.”

Tesla, which has disbanded its press offices, did not respond to a request for comment.

NHTSA said it has reports of 16 crashes, including seven injury incidents and one death, involving Tesla vehicles in Autopilot that struck stopped first-responder and road maintenance vehicles.

NHTSA said its analysis indicated that Forward Collision Warnings activated in the majority of incidents just prior to impact and that subsequent Automatic Emergency Braking intervened in approximately half of the crashes.

“On average in these crashes, Autopilot aborted vehicle control less than one second prior to the first impact,” the agency added.

NHTSA noted that “where incident video was available, the approach to the first responder scene would have been visible to the driver an average of 8 seconds leading up to impact.”

The agency also reviewed 106 reported Autopilot crashes and said in approximately half “indications existed that the driver was insufficiently responsive to the needs of the dynamic driving task.”

“A driver’s use or misuse of vehicle components, or operation of a vehicle in an unintended manner does not necessarily preclude a system defect,” the agency said.

Separately, NHTSA has opened 35 special crash investigations into incidents involving Tesla vehicles in which Autopilot or other advanced systems were suspected of use involving 14 reported deaths since 2016, including a crash that killed three last month in California.

Tesla CEO Elon Musk, recently said employee headcount will rise over the next year, despite an earlier report that he planned to make cuts at his company over concerns about the economy.