[Vlad The Inhaler]

Why was Mueller investigating the Trump transition to the point of grabbing the cell-site location data for the brother of Trump's ed secretary to see when he was at Trump Tower

IMHO _ Trying to uncover any possible opening to put the squeeze on a friend or relation of trump insiders.

Looking for ways to put pressure on insiders to turn them into informers or witnesses against the President.

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[Freee-dame]

Why was Mueller investigating the Trump transition to the point of grabbing the cell-site location data for the brother of Trump's ed secretary to see when he was at Trump Tower

This is a good example of how wide-ranging the "two hop" sureillance allowance is with a FISA warrant. Every person that communicates with the person named in the FISA warrant (example: Carter Page) can legally be surveilled. Also every person that that first circle of people communicate with can also be surveilled. This means by any means of communication in the past, present and future. The entire attempt to entrap Trump is really evil.

If Carter Page was actually working with the FBI (which I think is possible), then he could have steered his own communications to as many people as possible for the FBI to "legally" SPY on.