When Maine enacted legislation last week banning law enforcement officials from tracking individuals using cellphones or other GPS-enabled devices, it became the second state to do so after Montana. A similar effort failed in the Texas Legislature, but there is little doubt that other states will also act if Congress fails to update the statutes that govern access to digital communications.

One thing driving the states’ push for clear legal standards is the uncertainty of the current law. Location data is not explicitly addressed by the 1986 Electronic Communications Privacy Act (PL 99-508), so instead law enforcement officials rely on elements of that law that govern stored communications and other statutes.

The law can also vary depending where officials go to get the data; a phone seized by police is subject to the Fourth Amendment, while location data stored by a wireless service provider would be governed by ECPA.

Carriers often do not store GPS location data for individual users and are unable to provide it. But law enforcement officials frequently ask carriers to start compiling GPS data on individuals under investigation, and the Justice Department typically seeks a warrant for these cases. That’s in part because GPS data is recognized as highly private because it offers granular insight into an individual’s movements.

Providers, however, often keep records of a second type of location data, which is less precise but can be used as an effective means of tracking a phone’s location. Phones that place calls or use data networks must use a nearby cell tower, and the location of the towers that a particular phone has pinged can lend insight into a user’s movements.

For historical location records based on cell tower data, law enforcement seeks a court order, with a lower legal standard than the probable cause required for a warrant. For prospective location information based on cell tower locations, officials rely on a hybrid order involving several statutes.

Some courts have found that hybrid order unsatisfactory and have mandated a warrant for all prospective location data. A number of pending cases are being litigated in federal courts that would test the legality of the court orders, but experts believe legislation would be needed to impose a new warrant requirement.

Regardless, if Congress doesn’t act, it is clear that many states are comfortable taking matters into their own hands.