Law-enforcement officers who track the locations of people via their cell phones may be operating outside the bounds of a recent U.S. Supreme Court decision, Sen. Al Franken, D-Minn., said Thursday.

The Supreme Court ruled in January that placing a GPS device on a suspect's car constitutes a search under the U.S. Constitution's Fourth Amendment, a decision that helped define the scope of personal privacy in an age of ubiquitous GPS technology.

Based on the implications of that ruling, Franken sent a letter to U.S. Attorney General Eric Holder asking for more information on how frequently Justice Department officials request wireless carriers to provide information about the location of customers. That kind of data is easily obtained using a cell phone's GPS record.

Franken made his request for information in response to an American Civil Liberties Union report that noted law-enforcement agencies often go directly to wireless carriers to get location data. The senator wrote that doing so may ignore the legal precedent set in the Supreme Court case, U.S. v Jones, which he called a watershed for Americans' privacy and civil liberties.

I was very concerned to read recent reports suggesting that state and local law-enforcement agencies may be working around the protections of Jones, Franken wrote, adding that he was further concerned to learn that in many cases, these agencies appear to be obtaining precise records of individuals' past and current movements from carriers without first obtaining a warrant for this information.

Franken contended, I think that these actions may violate the spirit if not the letter of the Jones decision.

Among other things, Franken asked what sort of legal standard the Justice Department applies to obtaining cell-phone location data -- whether it requires a warrant, subpoena, or court order, for example -- and he asked whether the Jones decision has changed its practices at all.

The Jones ruling was actually a fairly narrow one that applied to the specific scenario of placing a device on someone's car, which the court said is a form of trespassing. The decision left unanswered larger questions about the intersection between personal privacy and powerful surveillance technology now available to the government.

Supreme Court Justice Samuel Alito chastised the majority in a concurring opinion, saying the justices had restricted themselves to only considering physical trespassing as he wondered whether the GPS tracking itself was an unreasonable search under the Fourth Amendment. He wrote that he found it almost impossible to think of late 18th-century situations that are analogous to what took place in this case.