By Devlin Barrett 

WASHINGTON--A Justice Department official defended the U.S. Marshals Service Friday after The Wall Street Journal revealed the agency operates a secret cellphone surveillance program that deploys an airborne device mimicking cell towers to cull registration data from thousands of Americans' mobile phones at a time.

Without formally acknowledging the existence of the program, the official said the Marshals Service doesn't maintain a database of everyday Americans' cellphones.

The official said any of the Marshals' investigative techniques are deployed "only in furtherance of ordinary law enforcement operations, such as the apprehension of wanted individuals, and not to conduct domestic surveillance or intelligence-gathering." The official said the agency doesn't maintain any databases of cellphone information of the general public, and said any activity is legal and "subject to court approval."

The statement came in response to a Wall Street Journal article Thursday night that revealed a secret program run by the U.S. Marshals Service to hunt fugitives by scanning the skies for cellphone registration numbers. The program, which has been fully operational since about 2007, sends up a small plane equipped with an airborne device which tricks cellphones into thinking the device is the nearest cell tower, according to people familiar with the program.

The phones, sometimes tens of thousands of them, will then send their unique identifying information to that device, which then sifts through them to find a suspect phone, these people said. As soon as it registers a phone that doesn't match the suspect's device, it "lets go" of that phone, these people said.

Once it finds the targeted phone, the plane continues to circle until it can tell the precise location of that individual to within 10 feet, these people said.

A Justice Department official on Friday said they still wouldn't confirm or deny the existence of such a program, because doing so would allow criminals to better evade law enforcement. But the official said it would be "utterly false" to conflate the law-enforcement program with the collection of bulk telephone records by the National Security Agency, a controversial program already being challenged in the courts and by some members of Congress.

Privacy advocates call the program a dragnet and have called into question whether judges who sign court orders are told the method being used to hunt for suspects. Because those court applications are sealed, it is unclear what courts have been told about the program.

One maker of the devices that mimic cell towers is a Boeing Co. subsidiary, which has stated in a public regulatory filing that some of their machines have the ability to store the numbers of non-targeted cellphones for later cross-referencing. And a person familiar with the technology being deployed said that data is stored when it is used overseas by U.S. military and intelligence personnel.

Write to Devlin Barrett at devlin.barrett@wsj.com

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