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.

They can get a warrant.

And by doing this they stop a terror attack?

Fuck off. This is just trying to manipulate people with fear of undefined bad thing.

You can justify anything and everything, including torturing random innocent civilians for information, under the guise of preventing terror attacks. Which is why it is a bullshit excuse.

What if we put cameras and sensors in every home? What if we require groups of three or more to register their gathering with the government?

What if we could torture someone to have a chance at stopping a terror attack? What if we could torture someone to find where they stashed a stolen car? What if publicizing the errant torture of innocent people is bad for public morale, so we outlaw publishing stories about it?

When does it stop?

These are basic philosophy of law questions but I tend to stand on the side of liberty from an ever more powerful government.