Fans of “The Wire,†the HBO series, will recall what a gold mine cellphones turned out to be for police investigating a drug ring in Baltimore. Detectives in the show used them to construct a map of who called whom at what time and how often.
Indeed, a list of incoming and outgoing calls on an individual's cellphone can provide a robust trail of evidence.
Cellphones seem to be increasingly attractive to the Department of Justice, documents obtained by the American Civil Liberties Union show. Agencies affiliated with the department used more than 37,600 court orders in 2011 to gather cellphone data, a sharp increase from previous years. They were almost equally divided between “pen register†data, which captures outgoing phone numbers, and “trap and trace†orders, which refer to incoming phone numbers, which means one phone could have two separate orders associated with it.
The total number has roughly doubled since 2007, when cellphone commu nications were more limited.
By law, the data can be obtained without a search warrant establishing probable cause, though the authorities do need to tell a court that it is relevant to an investigation. To get a wiretap that allows authorities to actually listen in on the contents of a call has higher legal barriers; law enforcement officials have to convince an impartial judge of probable cause.
The lower legal threshold allows law enforcement agencies to capture crucial information, including the time and date of calls and their length, helping law enforcement officials deduce important associations among callers. Each order, the A.C.L.U. pointed out, could affect one or more individuals.
Pen register orders can also allow law enforcement number to obtain data about e-mails, like the “to†and “from†fields, though not the content of those communications.
Among the total orders, the United States Marshals Service led the pack, with more tha n 16,000, followed by the Drug Enforcement Agency and the Federal Bureau of Investigation. The Justice Department, unlike local police, is required to report how many such orders it seeks. Still, the A.C.L.U. said it had to file a Freedom of Information Act request to obtain the latest figures.
