This week a federal court ruled that evidence collected by use of a Stingray was inadmissible where a warrant was not obtained. Stingrays are devices that mimic cell phone towers. They allow government agents to track the whereabouts of cell phones without the knowledge of the cell phone users. It is unknown how many agencies employ the use of Stingrays, because they also promise to keep them secret when they acquire them.
The DOJ issued a policy that their agents are supposed to get warrants before using the devices. That was a smart move, predicting the legal outcome when the Stingray evidence was challenged. This investigation occurred before that change in policy, and if the Feds had continued to collect this evidence without warrants… a lot more cases would be in Jeopardy. The DOJ policy does not govern local law enforcement agencies, who stand to have a lot of evidence in jeopardy if they have not been obtaining warrants, in light of this Federal Court decision.