The US National Highway Traffic Safety Administration (NHTSA) is investigating two rear-end crashes involving Amazon-owned Zoox self-driving cars and motorcycles.

The case was opened on Friday with an Office of Defects Investigation (ODI) preliminary evaluation that summarizes the circumstances of the two crashes, both of which involved a Zoox-powered Toyota Highlander braking suddenly and then being rear-ended by a motorcycle. The two motorcyclists only received minor injuries in the crashes.

“ODI has opened a Preliminary Evaluation to investigate the Zoox Automated Driving System equipped on the subject vehicles,” the notice [PDF] says. “This investigation will evaluate the Zoox Automated Driving System performance, particularly relating to the collisions listed in this resume, the behavior in crosswalks around vulnerable road users, and in other similar rear-end collision scenarios.”

The preliminary evaluation says there are 500 Zoox cars out on the road, though it’s not clear if this figure includes only Toyota Highlander models or if the self-driving car company‘s custom robotaxis are also counted among those vehicles.

The NHTSA has kept tabs on other self-driving and driving assisted companies. This past December the NHTSA’s Tesla investigation resulted in a software recall of two million cars, and last month the agency started looking into two fatal crashes that involved Ford’s BlueCruise self-driving cars. Both investigations explicitly concerned autonomous driving features, indicating that the NHTSA doesn’t take self-driving tech at face value.

Given the scope of the investigation, it may be some time before the NHTSA publishes its findings. The Tesla probe took two years to complete, albeit involving several different car models.

We’ve asked both Zoox and the NHTSA for comment. The federal agency declined to comment while Zoox hasn’t responded. ®