A federal judge just ruled that police cannot collect the location data of everyone near a crime scene just to see whether one of them might be the suspect.

The case involved cell towers, not Flock cameras.

But the warning applies more broadly.

A tower dump collects every phone near a location.

A Flock network records every license plate that passes a camera.

In both cases, the government can collect the movements of innocent people first and search for suspicion later.

That reverses the traditional order of an investigation.

Police are supposed to identify a suspect, establish probable cause and then search.

Mass surveillance builds the database first.

The legal rules for license plates and cellphone data are not identical. But the constitutional danger is the same:

When the government can search everyone, everyone becomes a suspect.

As the court put it:

“The Government cannot have access to an entire haystack because it may contain a needle.”