The ubiquitous cellphone has many functions in everyday life. It allows us to take pictures, make calls, send email and text messages. But for police, cellphones serve an entirely different purpose: tracking the geographic location of a cellphone user in connection with a criminal investigation.

The widespread nature of the practice became evident this week when the state’s top prosecutor said his office has been monitoring geographic location data from hundreds of Mainers’ cellphones during various criminal probes.

Deputy Attorney General William Stokes revealed the information Wednesday to justify his opposition to a bill requiring police to obtain a warrant to gain access to individuals’ cellphone location records. With such extensive tracking taking place, he estimated it would cost more than $500,000 over four years to monitor compliance with a controversial provision that police tell people within three days if they’ve been tracked via cellphone.

Stokes’ announcement comes at a critical time for L.D. 415, which could live or die for lack of state funds although it has initial legislative approval. But lawmakers shouldn’t let his arguments sway them. It’s easy to see why a prosecutor wouldn’t want to give up access to information that might solve a case, but that’s not the only issue at stake here. Maine could be the first state in the country to implement and fund constitutional protections to cellphone location data, and legislators shouldn’t miss this chance to make history.

Critics of L.D. 415 see the bill as another drain on the state budget. Two new state workers would be needed to track state police compliance with the notification mandate, Stokes predicts. But state police already have to notify people that they intend to search that person’s home, car or email account. Presumably, the state monitors compliance with this mandate as a matter of course. Tracking whether state police are following the notification mandate should be just as routine, and the Attorney General’s Office should be able to absorb the costs in its existing budget.

The bill recognizes the sensitive nature of criminal investigations and allows reasonable exemptions. If there’s a threat of imminent bodily harm, police don’t have to get a warrant to track a cellphone user’s location. There’s also an exception to the notification mandate: Police can ask a judge to delay notification for up to 180 days should the disclosure put an investigation at risk.

We don’t dispute the value of cellphone location data to state, county or local police investigators in Maine. This bill wouldn’t stop them from accessing this data. All it does is ensure that police get the go-ahead from a judge — a neutral party — to use someone’s cellphone records to track his or her location. And they’ll have to have probable cause to believe they can find evidence that the cellphone’s owner committed a crime. This constitutional protection applies to searches of cars and houses, and it should apply to searches of cellphone records, too.

The need for court oversight of the official use of personal data is already significant. In 2011 alone, major cellphone carriers received 1.3 million subscriber data requests from law enforcement officials. Recent revelations about the Justice Department’s seizure of journalists’ phone records add to the issue’s urgency. Maine shouldn’t pass up the opportunity to put in place privacy protections that an electronically connected world has made more critical than ever.

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