DETROIT (AP) — More than 750 Tesla owners have complained to U.S. safety regulators that cars operating on its partially automated driving systems have suddenly stopped on roadways for no apparent reason.

The National Highway Traffic Safety Administration revealed the number in a detailed information request letter to Tesla that was posted Friday on the agency’s website.

The 14-page letter dated May 4 asks the automaker for all consumer and field reports it has received about false braking, as well as reports of crashes, injuries, deaths.

It also asks whether the company’s “Full Self Driving” and automatic emergency braking systems were active at the time of any incident.

The agency began investigating phantom braking in Tesla’s Models 3 and Y last February after getting 354 complaints.