Or who's cell phone shut off 1 or 2 cells over then turned back on 1 or 2 cells over in the time frame of the crime. This is the kind of FBI sleuthing I'd support, assuming they got a warrant.
[0] http://www.nytimes.com/2014/10/26/us/law-lets-irs-seize-acco...
http://www.nytimes.com/2015/02/15/magazine/the-hezbollah-con...
If they're smart they'd want to minimize the electronic trail they'd leave behind and ditch the phone completely.