Amazon’s Zoox is now the brand of self-driving taxis under the regulatory microscope.

The National Highway Transportation Safety Administration is investigating two rear-end crashes that involved Zoox-equipped vehicles, both Toyota Highlanders with the brand’s automated driving system installed and engaged.

The U.S. transportation department agency said that in both incidents, the vehicles “unexpectedly braked suddenly,” resulting in a motorcycle behind them crashing into them and causing minor injuries. Both accidents happened during the daytime.

NHTSA is investigating the performance of the automated driving system installed on the vehicles involved in the accidents, along with how it operates in other similar rear-end collision situations and in crosswalks.

The accidents follow incidents involving General Motors’ Cruise taxis. NHTSA opened an investigation of that brand’s fleet in October to determine whether the taxis were “exercising appropriate caution around pedestrians in the roadway” after reports that some had “encroached” on pedestrians in or entering roads. The same month, California suspended Cruise’s deployment and permits in the state.

Studies show the public tends to be wary of driverless taxis and full vehicle automation technology in general, and a technology association recently cast doubt on the idea of such technology ever being widely used on American roads.