U.S. safety regulators have opened a probe into Amazon-owned robotaxi venture Zoox after two of its autonomous SUVs braked suddenly and were rear-ended by motorcyclists.

The National Highway Traffic Safety Administration said Monday it initiated an investigation after two Toyota Highlanders equipped with Zoox’s automated self-driving braked unexpectedly. In both cases, motorcyclists collided with the Zoox vehicles, which led to minor injuries.

NHTSA said it confirmed that each Zoox car had been operating in autonomous mode when the incidents occurred. Both collisions took place during the daytime and within the operational design limits of Zoox’s autonomous system.

The agency’s probe covers about 500 vehicles and focuses on the performance of Zoox’s automated driving system, specifically how it behaves near crosswalks and “in other similar rear-end collision scenarios.”

Representatives from Zoox did not immediately respond to a request for comment.

Amazon acquired Zoox for a reported $1.2 billion in 2020. Zoox, which has largely continued to operate as a standalone unit within Amazon, has been developing an electric, fully driverless vehicle designed for ride-hailing. Zoox has been testing its robotaxis in Las Vegas, Nevada, and Foster City, California. In March, the company said it was expanding its operating routes in those areas to include a larger area, faster speeds, nighttime driving and light rain conditions.

This latest news marks at least the second probe into Zoox by the NHTSA. In March 2023, the agency said it would investigate Zoox’s 2022 self-certification that its robotaxi met federal safety standards.