In this article, the author explains how for decades the FBI has used a questionably-legal (I'd say illegal) tactic of creating fake cell-phone networks by means of a "Stingray," which the FBI then uses to monitor surveillance in a region. The FBI does not secure warrants to deploy Stingrays, saying that they only use them as a “Pen register” to collect metadata. However, these Stingrays, once deployed, gather data for everyone in the area. It is unclear how many of these are currently deployed around the US...

-- SamuelDostart - 16 Feb 2013