The U.S. Department of Justice is putting devices that emulate cellphone towers in Cessna aircraft and flying them around the country to track the locations of cell phones, a practice that targets criminal suspects but may also affect thousands of U.S. citizens, according to a news report Thursday.

The program is run by the Department of Justice’s U.S. Marshals Service and has been in operation since at least 2007, according to the report in the Wall Street Journal, which cited two unnamed sources. The aircraft are flown out of at least five metropolitan-area airports and can cover most of the U.S. population, it said.

Cell phones are programmed to connect to whichever nearby cell tower has the strongest signal. The fake cell towers trick phones into thinking they have the strongest signal, then read the devices’ unique registration numbers when they connect, the Journal report says.

The goal is to locate cell phones linked to people under investigation for crimes like selling drugs, but in the process the program collects data about people not suspected of any crime, the report says. The fake cell towers determine which phones belong to criminal suspects and “let go” of those that aren’t.

The Journal quoted a representative of the American Civil Liberties Union who called it an inexcusable “dragnet surveillance program.”

A DOJ official wouldn’t confirm or deny the program but said Justice Department agencies “comply with federal law, including by seeking court approval,” the Journal said.

A side effect is that the towers can sometimes cause cell phone calls to be dropped. “Authorities have tried to minimize the potential for harm, including modifying the software to ensure the fake tower doesnt interrupt anyone calling 911 for emergency help,” one source told the Journal.

There have been several reports in the last few months about phony cell phone towers being detected around the U.S. It’s not clear whether the fake cell towers used by the DOJ, dubbed “dirtboxes” by law enforcement officials, are the ones that have been detected.