More than 750 Tesla drivers told regulators their cars randomly slammed on the brakes while driving

• The US is stepping up its investigation into a problem with Tesla vehicles known as "phantom braking."
• Over 750 drivers have submitted complaints saying their Teslas unexpectedly hit the brakes at highway speeds.

The US government is stepping up an investigation into Tesla after more than 750 drivers complained that their cars suddenly slammed on the brakes at high speeds.

The National Highway Traffic Safety Administration has received 758 reports from Tesla owners who say they've experienced a dangerous phenomenon known as "phantom braking," according to a letter the agency sent to Tesla on May 4 and made public on Friday.

In complaints to NHTSA, Tesla owners recounted frightening incidents when their vehicles unexpectedly and violently hit the brakes at highway speeds. Drivers said phantom-braking incidents typically happened when they had switched on cruise control or Autopilot, a system that automatically accelerates, brakes, and steers on highways.

The problem could be an issue with Tesla's automatic emergency braking system, a safety feature designed to detect a imminent crash and stop the car. Some drivers said shadows and oncoming cars seemed to trigger phantom-braking events. Some said they had to take evasive action to avoid getting rear ended after slowing down.

NHTSA said in February that it is investigating the issue in Model Y SUVs and Model 3 sedans from the 2020 and 2021 model years. In the May letter, the agency requested that Tesla respond to a long list of questions by June 20. The automaker faces penalties of up to $115 million if it doesn't respond in time.

Tesla is facing mounting scrutiny from regulators, particularly over its semi-autonomous driving technology. The US is investigating incidents where Tesla vehicles on Autopilot or cruise control crashed into stopped emergency vehicles.

Are you a Tesla owner with a story to share about phantom braking or anything else? Are you a Tesla employee with a tip? Contact this reporter at tlevin@insider.com