The US government’s highway safety agency (NHTSA) has opened another investigation of automated driving systems, this time into crashes involving Waymo’s self-driving vehicles.

NHTSA posted documents detailing the probe on its website yesterday 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, says 14 News in Detroit.

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.

Waymo said NHTSA plays an important role in road safety, and it will continue working with the agency “as part of our mission to become the world’s most trusted driver.”

The company said it makes over 50,000 weekly trips with riders in challenging environments. “We are proud of our performance and safety record over tens of millions of autonomous miles driven, as well as our demonstrated commitment to safety transparency,” the statement said.

Waymo, based in Mountain View, California, has been operating robotaxis without human safety drivers in Arizona and California.