You get a "geofence warrant." They exist and are ubiquitous. You then go to Google or any other provider and you demand the data for a specific location in a specific time window. You then use the data to capture criminals. Any other data would not meet the standards of evidence and probably couldn't be used in court anyways. It's only function is for "parallel construction."

Then again, what I _really_ want is for the FBI to prevent crime. If their only solution is to let crime happen and then use a giant dragnet to put people in jail then they are less than worthless... they are actively dangerous to democracy.