From ZDNET.com:
The FBI appears to have begun using a novel form of electronic surveillance in criminal investigations: remotely activating a mobile phone's microphone and using it to eavesdrop on nearby conversations."
Since the FBI was called in to early in the case, I wonder if they turned on the missing phones microphones and listened? I love all this new technology.
Please see the search warrant application. The phones had not been located as of mid-October.