A federal appeals court in Philadelphia is the first circuit to weigh the issue, in a closely watched case being argued Friday.
The court must decide whether warrants are needed to obtain cell-phone company data that can point to an individual's location.
A magistrate in Pittsburgh denied the request in a 2008 drug-trafficking case, prompting the Justice Department's appeal. The American Civil Liberties Union and other privacy groups oppose the tactic.
Magistrate Lisa Pupo Lenihan calls the information extremely personal, because it can reveal medical visits, extramarital affairs or other sensitive information.
