A Waymo autonomous vehicle pulled over in San Francisco after the company detected a suspected firearm-related violation involving two juvenile passengers, prompting police to respond and make arrests.

The incident occurred around 4 a.m. Sept. 3, after Waymo detected what the Los Angeles Times described as a violation of its terms of service involving a firearm. The company contacted authorities and “cooperated fully” with the San Francisco Police Department during the investigation.

Police said they received a report of two passengers inside an autonomous vehicle with a firearm. When officers arrived and searched the vehicle, they found what the department described as a loaded AR-style rifle, which police identified as a “ghost gun.”

Officers also allegedly recovered marijuana and mace spray from one of the suspects. Both passengers were taken into custody and booked, but their names were not released because they are juveniles.

The incident highlights one of the unusual challenges facing autonomous ride-hailing services as the vehicles increasingly operate without human drivers. Waymo’s systems can detect certain situations inside or around its vehicles and allow the company to respond when a ride appears to violate its policies or present a safety concern.

The San Francisco incident was not the first time a Waymo vehicle has responded to unusual passenger behavior. Earlier this year, another Waymo vehicle reportedly simulated mechanical problems and pulled over after two passengers were allegedly drinking alcohol and firing a toy gun from the back of the vehicle.

Waymo’s decision to stop the vehicle and contact police in the latest incident resulted in officers locating the firearm and taking both juvenile passengers into custody.