SAN FRANCISCO — Seems like Uber is in another tussle with the California Department of Motor Vehicles over its self-driving vehicle testing. And this time it's trucks.

A few months ago, the ride-hailing giant announced that it would begin testing self-driving Volvo SUVs in this hilly city, but a day later that process was halted after the DMV said Uber had not applied for the proper permits. Uber moved its fleet to Arizona.

Uber cars laden with sensors still troll San Francisco, but the company said it is only for mapping purposes.

Now a southern California non-profit that has long raised concerns about the safety of autonomous vehicles has asked the DMV to look closer at the operations of Otto, a self-driving truck company that Uber bought last year for $670 million.

Prior to that event, Otto invited USA TODAY to ride in its Volvo trucks in San Francisco, a short drive that mainly saw the vehicle stick to its lane while a driver watched over the controls. The company noted that this did not officially constitute self-driving mode.

But Consumer Watchdog's John Simpson charged in a letter to DMV director Jean Shiomoto that in fact Otto's testing here did violate the law by operating in autonomous mode, offering proof in the form of documentation Otto submitted to Colorado officials that described a process where the driver hit a button and let the truck do the work.

"There is no more explicit description of how self-driving testing is performed," wrote Simpson. "Otto is simply doing this in violation of the law."

Otto staff met with the DMV and the California Highway Patrol and indicated that its trucks are not capable of operating in autonomous mode in California, the DMV told USA TODAY. Since then, the Department has received a copy of the Colorado document and is looking into the issue, it said.

Uber and Otto officials declined to comment