Amazon’s self-driving robotaxi unit is being investigated by the U.S. government’s highway safety agency after two of its vehicles braked suddenly and were rear-ended by motorcyclists.

The National Highway Traffic Safety Administration said in documents posted on its website Monday it will evaluate the automated driving system developed by Zoox.

Both crashes happened during daytime hours, and the motorcyclists suffered minor injuries. In both cases, the agency confirmed that the Amazon vehicles were operating in autonomous mode leading up to the crashes.