The full capablities of the 'Stingray' phone tracker used by law enforcement is shrouded in secrecy..

For more than a year, federal authorities pursued a man they called simply "the Hacker." Only after using a little known cellphone-tracking device—a stingray—were they able to zero in on a California home and make the arrest.

Stingrays are designed to locate a mobile phone even when it's not being used to make a call. The Federal Bureau of Investigation considers the devices to be so critical that it has a policy of deleting the data gathered in their use, mainly to keep suspects in the dark about their capabilities, an FBI official told The Wall Street Journal in response to inquiries.

A stingray's role in nabbing the alleged "Hacker"—Daniel David Rigmaiden—is shaping up as a possible test of the legal standards for using these devices in investigations. The FBI says it obtains appropriate court approval to use the device.

Stingrays are one of several new technologies used by law enforcement to track people's locations, often without a search warrant. These techniques are driving a constitutional debate about whether the Fourth Amendment, which prohibits unreasonable searches and seizures, but which was written before the digital age, is keeping pace with the times.

A stingray works by mimicking a cellphone tower, getting a phone to connect to it and measuring signals from the phone. It lets the stingray operator "ping," or send a signal to, a phone and locate it as long as it is powered on, according to documents reviewed by the Journal. The device has various uses, including helping police locate suspects and aiding search-and-rescue teams in finding people lost in remote areas or buried in rubble after an accident. 

The government says "stingray" is a generic term. In Mr. Rigmaiden's case it remains unclear which device or devices were actually used.

The best known stingray maker is Florida-based defense contractor Harris Corp. A spokesman for Harris declined to comment.

Harris holds trademarks registered between 2002 and 2008 on several devices, including the StingRay, StingRay II, AmberJack, KingFish, TriggerFish and LoggerHead. Similar devices are available from other manufacturers. According to a Harris document, its devices are sold only to law-enforcement and government agencies.

FBI and Department of Justice officials have also said that investigators don't need search warrants. Associate Deputy Attorney General James A. Baker and FBI General Counsel Valerie E. Caproni both said at a panel at the Brookings Institution in May that devices like these fall into a category of tools called "pen registers," which require a lesser order than a warrant. Pen registers gather signals from phones, such as phone numbers dialed, but don't receive the content of the communications.

To get a pen-register order, investigators don't have to show probable cause. The Supreme Court has ruled that use of a pen register doesn't require a search warrant because it doesn't involve interception of conversations.

But with cellphones, data sent includes location information, making the situation more complicated because some judges have found that location information is more intrusive than details about phone numbers dialed. Some courts have required a slightly higher standard for location information, but not a warrant, while others have held that a search warrant is necessary.

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