The Justice Department is collecting data from thousands of cellphones through high-tech gear deployed on airplanes that mimics communications towers, The Wall Street Journal reported Thursday.

The newspaper said the hunt for information about criminal suspects is also collecting data from many innocent Americans.

Citing sources familiar with the operations, the newspaper said the U.S. Marshal's Service program, which became fully operational in 2007, operates Cessna aircraft from at least five metropolitan-area airports to collect the data. The airports were not identified in the Journal story.

The planes are equipped with devices that mimic cell towers of large telecommunications firms and trick cellphones into reporting unique registration information. The 2-foot-square devices allow investigators to collect data from thousands of cellphones in a single flight, the Journal reported. The devices collect their identifying information and general location.

The Justice Department would neither confirm nor deny the existence of such a program to the Journal. An official told the newspaper that discussion of such matters would allow criminal suspects of foreign powers to determine U.S. surveillance capabilities, adding that Justice Department agencies comply with federal law, including by seeking court approval.

Calling it "a dragnet surveillance program," Christopher Soghoian, chief technologist at the American Civil Liberties Union, said: "It's inexcusable and it's likely--to the extent judges are authorizing it--they have no idea of the scale of it."

The device being used by the Marshals Service identifies itself as having the closest, strongest signal--though it doesn't--and causes all the cellphones that can detect its signal to send in their unique registration information. Cellphones are programmed to connect automatically to the strongest cell tower signal.

Phone companies are cut out in the search for suspects. Law enforcement has found that asking a company for cell-tower information to help locate a suspect can be slow and inaccurate. This program allows the government to get that information itself.

People familiar with the program told the Journal they do get court orders to search for phones, but it isn't clear whether those orders describe the methods used because the orders are sealed.