More than 750 Tesla owners have complained to US safety regulators that cars operating on the automaker’s 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.

Shares of Tesla tumbled 8% in midday trading.

A message was left early Friday seeking comment from Tesla.

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 said.

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

In the letter, NHTSA asks for the initial speed of when the cars began to brake, the final speed, and the average deceleration. It also asks if the automated systems detected a target obstacle, and whether Tesla has video of the braking incidents.

The agency is now seeking information on warranty claims for phantom braking including the owners’ names and what repairs were made. It’s also seeking information on Tesla’s sensors, any testing or investigations into the braking problems, or if any modifications were made.

The letter focuses on Tesla’s testing of the automated systems when it comes to detecting metal bridges, s-shaped curves, oncoming and cross traffic, and different sizes of vehicles including large trucks. The agency also wants information on how cameras deal with reflections, shadows, glare and blockage due to snow or heavy rain.

The agency asks Tesla to detail its assessment of the “alleged defect” in the automated systems, including what caused the unnecessary braking, what failed, and the risk to motor vehicle safety that the problem poses. It asks Tesla “what warnings, if any, the operator and the other persons both inside and outside the vehicle would have that the alleged defect was occurring, or subject component was malfunctioning.”

The probe is another in a string of enforcement efforts by the agency that include Autopilot and “Full Self-Driving” software. Despite their names, neither feature can drive the vehicles without people supervising.

It’s the fourth formal investigation of the Texas automaker in the past three years, and NHTSA is supervising 23 Tesla recalls since January 2021.

Tesla and CEO Elon Musk have been fighting with US and California government agencies for years, sparring with NHTSA and most notably with the Securities and Exchange Commission.