A fraud case in Arizona raises the issue of whether police need a search warrant to use "stingrays," devices that enable them to locate people's mobile phones by mimicking cell towers. The FBI used a stingray to find Daniel David Rigmaiden, who is accused of stealing $4 million by filing phony online tax returns. It obtained two court orders authorizing use of the device to zero in on his mobile broadband card. One was a pen register/trap and trace order, which requires only that a law enforcement agency certify that the surveillance is relevant to an investigation. The classic use for such orders is obtaining the phone numbers dialed by a target or the numbers from which he receives calls, information the Supreme Court says is not protected by the Fourth Amendment because you voluntarily relinquish it to the phone company. (The restrictions for this kind of surveillance, such as they are, were imposed by statute.) The FBI says it also obtained a search warrant to use the stingray, based on a showing of probable cause. Rigmaiden's lawyers argue that the order was not a proper search warrant becase it did not specify the technology to be used and allowed the FBI to delete the information it collected. In any case, the FBI and other law enforcement agencies maintain that a search warrant is not legally required to use a stingray.
The legal question is whether stingrays—which can track someone to his home (as in Rigmaiden's case) or to other places where he has a reasonable expectation of privacy—is more akin to a pen register or trap and trace device, which can be used without a warrant, or to a wiretap, which requires a warrant. It seems strange that demanding someone's records from a third party would require less of a showing than using a device that requires no cooperation or physical intrusion. Yet that is the implication of the Supreme Court's 2001 ruling in Kyllo v. U.S., which held that police need a warrant to monitor the infrared energy emitted by a home. Using a stingray is analogous to thermal imaging in the sense that both can reveal information about what is going on inside people's houses, a domain the Fourth Amendment specifically mentions. (In fact, a stingray is arguably more intrusive, since it works by drawing signals from phones, as opposed to passively measuring heat.) But the Fourth Amendment also mentions "papers," a concept that should cover information we entrust to third parties for specific purposes. Because the Supreme Court has said the Fourth Amendment does not apply to such information, it receives only the protection that Congress decides to give it.
[Thanks to Tricky Vic for the tip.]