It's the second investigation this week into robotaxis, just a day after the National Highway Traffic Safety Administration started a review into Foster City-based self-driving company Zoox.
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Now, they are announcing Tuesday morning a probe into Waymo.
VIDEO: Waymo robotaxi shown breaking bus lane laws in San Francisco
Video shows Waymo robotaxi breaking bus lane laws in SF
It comes after 22 reports of Waymo vehicles either crashing or doing something that may have violated traffic laws.
In the crashes, the vehicles hit stationary objects like gates, chains or parked vehicles.
Some of the incidents happened right after the Waymo driving system behaved unexpectedly near traffic signs and cones.
The agency said it understands that Waymo's automated driving system was engaged in each incident.
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MORE: Amazon's self-driving robotaxi unit Zoox under investigation by US after 2 rear-end crashes
In some cases involving a test vehicle, a human driver disengaged the system just before an accident happened.
The company said it makes over 50,000 weekly trips with riders.
Waymo is based in Mountain View and has been operating robotaxis without human safety drivers in Arizona and California, including in San Francisco.