The National Highway Traffic Safety Administration has announced that it has opened an investigation after a self-driving Waymo vehicle was caught passing a stopped school bus in metro Atlanta last month.

Video shared on social media showed the Waymo vehicle passing a school bus that had stopped with its red lights flashing and stop arm deployed. Students were disembarking at the time.

In their summary of the situation, the NHTSA reports that the Waymo AV initially stopped, but then drove around the bus, turning to avoid hitting the vehicle's front end before turning left to pass it.

"During this maneuver, the Waymo AV passed the bus's extended crossing control arm near disembarking students," the report reads.

The NHTSA's Office of Defects Investigation says it has opened a preliminary evaluation to see how Waymo's system handles stopped school buses.

A spokesperson for Waymo tells CBS News that the bus's lights and stop sign were not visible to the autonomous vehicle at the time it was making the turn and maintained a slow speed to keep a safe distance from any children exiting the bus.

"Safety is our top priority, as we provide hundreds of thousands of fully autonomous paid trips every week in some of the most challenging driving environments in the U.S. The data shows we are improving road safety in the communities in which we operate, achieving a fivefold reduction in injury-related crashes compared to human drivers, and 12 times fewer injury crashes involving pedestrians," the Waymo spokesperson said. "NHTSA plays a vital role in road safety, and we will continue to work collaboratively with the agency as part of our mission to be the world's most trusted driver."

Atlanta police did not issue a citation in the case and told CBS News that they had not received a report on the incident.

Waymo expanding robotaxi services 

Waymo's self-driving taxis have been operating in the United States for years, and currently serve Atlanta as well as the cities of Phoenix, San Francisco, Los Angeles, and Austin. The company is currently testing its services in New York City and hopes to be in Washington, D.C., Miami, Dallas, London, and parts of Japan next year.

While the company touts its safety data showing 91% fewer serious injury crashes compared to human drivers, driving mistakes by the autonomous vehicles have gone viral multiple times.

On Sept. 27, police in San Bruno, California, pulled over a Waymo after it made an illegal U-turn. Officers said that since there was no human driver, a ticket could not be issued, saying their citation books "don't have a box for 'robot.'"

"Hopefully the reprogramming will keep it from making any more illegal moves," the San Bruno Police Department wrote on Facebook.

A Waymo spokesperson says the company has already made some software updates to improve the system's performance and is working to implement additional improvements.