The Justice Department says it will beef up legal requirements for using cell-site simulators, an increasingly controversial form of surveillance technology that secretly gathers data about mobile devices.

Under the new policy, federal investigators will be required to get a warrant from a judge demonstrating probable cause, in most domestic criminal probes. Agents will need to explain to judges how the technology is being used. And they'll be directed to destroy volumes of bystanders' data "no less than once daily."

"This policy is really designed to ... try to promote transparency, consistency and accountability, all while being mindful of the public's privacy interest," said Deputy U.S. Attorney General Sally Yates.

Yates said the devices, commonly known as StingRays or dirt boxes, have been critical in cracking drug cases, finding fugitives and determining the whereabouts of kidnap victims. She declined to provide numbers on how often cell-site simulators are used but said they're deployed in only "a fraction" of federal investigations in keeping with public safety.

The new policy applies to federal agents under Justice Department control and to state or local investigators who work on federal task forces. Yates also said she expects the FBI to modify the terms of its "nondisclosure" agreements with state and local entities, some of which have barred those law enforcement agencies from disclosing the use of the devices, even to criminal defendants.

Yates said the Justice Department is trying to balance privacy interests of citizens with a desire to solve crimes while "not giving a road map to the bad guys who are trying to defeat it." In emergency or "exigent" circumstances, agents may not need to secure a warrant, but approval of high-level attorneys will be required.

The federal action follows outcry from judges, privacy advocates and members of Congress over the intrusive nature of the technology, which mimics cellphone towers to collect identifying information from mobile phones in a particular area. The StingRays can sweep in high volumes of information from innocent people inside homes or parks. - Read More at New Cellphone Surveillance

New Cellphone Surveillance Safeguards Imposed On Federal Law Enforcement