DETROIT — The U.S. government’s highway safety agency has opened another investigation of automated driving systems, this time into crashes involving Waymo’s self-driving vehicles.

The National Highway Traffic Safety Administration posted documents detailing the probe on its website early Tuesday after getting 22 reports of Waymo vehicles either crashing or doing something that may have violated traffic laws.

In the past month, the agency has opened at least four investigations of vehicles that can either drive themselves or take on at least some driving functions as it appears to be getting more aggressive in regulating the devices.

In the probe of Waymo, which was once Google’s self-driving vehicle unit, the agency said it has reports of 17 crashes and five other reports of possible traffic law violations. No injuries were reported.

In the crashes, the Waymo vehicles hit stationary objects such as gates, chains or parked vehicles. Some of the incidents happened shortly after the Waymo driving system behaved unexpectedly near traffic control devices, according to the documents.

A message was left early Tuesday seeking comment from Waymo, based in Mountain View, California, which has been operating robotaxis without human safety drivers in Arizona and California.

The NHTSA said it would investigate the 22 incidents involving Waymo’s fifth generation driving system plus similar scenarios “to more closely assess any commonalities in these incidents.”

The agency said it understands that Waymo’s automated driving system was engaged throughout each incident, or in some cases involving a test vehicle, a human driver disengaged the system just before an accident happened.

The probe will evaluate the system’s performance in detecting and responding to traffic control devices, and in avoiding crashes with stationary and semi-stationary objects and vehicles, the documents said.

Since late April, NHTSA has opened investigations into collisions involving self-driving vehicles run by Amazon-owned Zoox, as well as partially automated driver-assist systems offered by Tesla and Ford.

In 2021 the agency ordered all companies with self-driving vehicles or partially automated systems to report all crashes to the government. The probes rely heavily on data reported by the automakers under that order.