The use of cellphone records to place suspects at or near crime scenes is coming under attack in courts nationwide, challenging an established practice by federal and local law enforcement that has helped lead to thousands of convictions, reports The Washington Post.

Cellphone records are often used as evidence, relied upon to trace which cell tower was used to make or receive a call and then determine a caller’s whereabouts. But experts say that using a single tower to precisely locate where someone was at the time of a crime has severe limitations.

In 2012, federal and local law enforcement agencies made more than 1.1 million requests for the personal cellphone data of Americans for a variety of investigative reasons, according to an annual privacy-related survey by Sen. Edward J. Markey (D-Mass.). But that may become more difficult. This month, a federal appeals court ruled for the first time that law enforcement agencies must obtain a search warrant for cellphone location data.

At the heart of the controversy over cellphone data is a debate about how cellphone calls are routed and the range of the cell towers with which the phones connect. The FBI and local police officials maintain that they can place a suspect in a particular area because a cellphone, when making or receiving a call, usually selects the closest tower with the strongest signal and that most towers have a range of no more than two miles.