Approximately one-in-four of the agencies Kelly surveyed have executed what is known as a "tower dump," which involves collecting phone records from wireless providers that show the identity, activity and location of any phone that's been picked up by a set of cell towers over a given time frame.

And at least 25 police departments own Stingrays — suitcase-sized devices that Kelly describes as "fake cell towers." They can be clandestinely driven into neighborhoods and "trick all nearby phones into connecting to it and feeding data to police." Some states allow any local department to use them, and the purchases, which cost up to $400,000, are usually funded by the government through anti-terror grants.