Amazon's self-driving robotaxi unit is being investigated by the US 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 the daytime hours, and the motorcyclists suffered minor injuries. In both cases, the agency confirmed that each of the Amazon vehicles was operating in autonomous mode leading up to the crashes.

The agency said the probe will focus on the performance of the company's automated driving system during the crashes, as well as how it behaves in crosswalks around pedestrians and other vulnerable road users.

A message was left early Monday seeking comment from Zoox.

Zoox reported the crashes under an order to automated vehicle companies issued in 2021.

Amazon acquired Zoox in June of 2020 for a price that analysts pegged at over USD1 billion. In 2023 the Foster City, California, company said one of its funky-looking four-person shuttles autonomously carried employees on public roads on a mile-long (1.6 kilometre) route between two Zoox buildings.

The company later planned to launch a shuttle service exclusively for its employees. Analysts say they expect Amazon to use the Zoox system for autonomous deliveries.

Zoox vehicles don't have a steering wheel or pedals. The carriage-style interior of the vehicle has two benches that face each other. It measures just under 12 feet (3.7 meters) long, about a foot (a third of a meter) shorter than a standard Mini Cooper and can travel up to 35 miles per hour (56 kph).

Zoox already was under investigation by NHTSA. In March of 2022 the agency began looking into the company's certification that its vehicle met federal safety standards for motor vehicles.

The agency said at the time that it would look into whether Zoox used its own test procedures to determine that certain federal standards weren't applicable because of the robotaxi's unique configuration.