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, an FBI official told The Wall Street Journal. 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. 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. Mr. Rigmaiden maintains his innocence and says that using stingrays to locate devices in homes without a valid warrant "disregards the United States Constitution" and is illegal.