The system consists of a camera and a computer that records license plates of passing vehicles for comparison against plates that officers are seeking. It dramatically increases officers' ability to monitor vehicles moving through a community.

Googins equated it to having 100 extra officers watching for certain license plates.

But with that technology comes the ability to track exponentially more than any officers can or would, and the potential for abuse.

Although South Portland police have sought to craft internal policies to ensure responsible use of the device, the law needs to be written for all police agencies in the state, Bellows said.

Civil libertarians worry that without controls, the devices could be used to track the movements of political protesters, government critics, elected officials or anyone known personally by an officer,such as a former spouse. The compromise would not allow such uses.

The MCLU had sought to ban the devices. Failing that, it wanted to require officers to have reasonable suspicion before searching for a particular license plate.

Police say they want to be able to look for potential witnesses to crimes, possible drunk drivers, or abusive partners who are under a judge's order to stay away from victims – none of which would meet the reasonable-suspicion test.

The agreement reached Thursday would allow such uses.

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