In a report, the company reveals the car was trying to navigate its way round some sandbags at 2mph
A Google driverless car has been involved in a collision with a bus in California.
The tech company has admitted at least partial responsibility for the accident which occurred on 14 February.
It is thought to have happened while the company was testing 24 of the cars fitted with sensors and cameras near the tech firm's Silicon Valley headquarters.
In a report filed with California's Department of Motor Vehicles (DMV) the company reveals the car was trying to navigate its way round some sandbags at 2mph when it hit the bus that was travelling at 15mph.
Google's driverless cars have been involved in more than a dozen collisions but in most cases the vehicles were rear-ended.
If it is determined Google's car was at fault over this crash it will be the first time one of its driverless vehicles has caused an accident while in autonomous mode.
Google says the accident came from the same kind of negotiations and misunderstandings that take place between human drivers every day.
The company says its cars will now have a better understanding that buses and other large vehicles are less likely to give way than smaller ones.
A DMV spokeswoman said the agency hoped to speak with Google soon about what went wrong.