Sara Morrison in Vox:
A few days after the Capitol insurrection last January, the FBI got two tips identifying an Ohio man named Walter Messer as a participant, and both cited his social media posts about being there. To verify those tips, the FBI turned to three companies that held a large amount of damning evidence against Messer, simply as a result of his normal use of their services: AT&T, Facebook, and Google. AT&T gave the FBI Messer’s telephone number and a list of cell sites he used, including one that covered the US Capitol building at the time of the insurrection, per the criminal complaint against Messer. Facebook told the FBI that the phone number provided by AT&T was linked to Messer’s Facebook account, where he posted several selfies from inside the Capitol during the riot.
Google gave the FBI precise location data showing Messer’s journey from Ohio to DC and back again between January 5 and 7, as well as his location on the afternoon of January 6 as he wandered around and ultimately inside the Capitol building. The complaint also lists videos of the riot posted on Messer’s YouTube channel, Messer’s YouTube searches, internet searches, and emails from his Gmail account — all used to help build a case against him.