The Guardian in Britain recently gained access to an unclassified report from the FBI that basically brainstorms potentially nefarious uses for the tech. The government agency has dreamed up all sorts of nightmare scenarios about how criminals might exploit cars no longer needing a driver. Many of them wouldn't even be possible given the current technological limits. Driverless vehicles "will have a high impact on transforming what both law enforcement and its adversaries can operationally do with a car," according to the report.
Among the FBI's biggest fears is simply that autonomous vehicles will allow drivers to take their hands off the steering wheel and not pay attention to the road. Imagine a high-speed chase where a criminal flips a switch into auto mode and can freely turn around and shoot at police. Furthermore, the agency worries that with some hacking, a driverless car could become a fantastic weapon. A terrorist could load one up with a bomb and send the model on its way with on one inside.
Hands-free driving is a double-edged sword, though, and the report also foresees autonomous cars making surveillance easier. Law enforcement officers could more easily tail a specific vehicle from farther away, and they would be unencumbered to take photos or video.
Obviously, we are still some years away from any of these fears becoming a reality. At the moment, truly autonomous vehicles are still in limited testing on public roads and can't even do some of the things that the FBI suggests. That's not keeping law enforcement from trying to stay one step ahead of the bad guys, though.