REPORT: US Spy Program Targets Americans' Cellphones

The surveillance program, which is run by the US Marshals Service under the DOJ, has reportedly been in effect since 2007.

Officials have been using portable cell towers, known as "dirtboxes," on small aircraft to collection identity and location information on cell phone users.

Those Cessna aircraft fly from at least five airports near major cities that allow the DOJ to collect information on American cellphone users.

It's unclear how often the Marshals Service has been going on these surveillance missions, but the Wall Street Journal's sources said they happen "on a regular basis."

The DOJ refused to confirm or deny the Journal's report, citing concerns over revealing U.S surveillance practices to those who might want to evade them.

Cellphones are constantly in communication with nearby cell towers. The boxes used by the program allow their planes to pose at the nearest cell phone tower, thus tricking phones into disclosing sensitive user data.

The dirtboxes also have the ability to interrupt calls, though officials have reportedly tried mitigate the harmful consequences of that function.

Using this practice, US officials can essentially locate somebody down to the room they're in.

The Journal's sources say that once the Marshals Service has located their target, the system "lets go" of other users' data, though it's not clear what happens to that data.