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 and property damage claims. 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. The probe covers an estimated 416,000 vehicles from the 2021 and 2022 model years. In February, the agency said it had no reports of crashes or injuries.

The letter gives Tesla a deadline of June 20 to respond to the information request but says the company can ask for an extension.

In opening the probe, the agency said it was looking into vehicles equipped with automated driver-assist features such as adaptive cruise control and “Autopilot,” which allows them to automatically brake and steer within their lanes.

“Complainants report that the rapid deceleration can occur without warning, and often repeatedly during a single drive cycle,” the agency says.

Many owners wrote in their complaints that they feared a rear-end crash on a freeway.