To nab a pair of men accused of robbing banks in Connecticut, the FBI turned to a novel investigative technique last year: warrantless monitoring of the locations of about 180 different cell phones.
The FBI obtained a secret order--it has not been made public--commanding nine different telephone companies to provide federal police "with all cell site tracking data and cell site locator information for all incoming and outgoing calls to and from the target numbers."
But because the U.S. Justice Department did not obtain a warrant by proving to a judge that there was probable cause to suspect criminal activity, there's now a risk that the evidence from the location surveillance may be tossed out of court as illegally obtained. (Here's a list (PDF) of the phone numbers tracked.)
For more on this story, read FBI used 'dragnet' warrantless cell tracking on CNET News.