From a WaPo article:
The Capitol, more than most buildings, has a vast cellular and wireless data infrastructure of its own to make communications efficient in a building made largely of stone and that extends deep underground and has pockets of shielded areas. Such infrastructure, such as individual cell towers, can turn any connected phone into its own tracking device.
Phone records make determining the owners of these devices trivially easy. Congressional investigators and federal prosecutors can also identify devices and users who may have connected wittingly or automatically to congressional guest WiFi networks — unless rioters made a point of deactivating their devices or leaving them behind during the takeover.
When engaging in illegal behavior, it's best to leave your mobile tracking device elsewhere. You know, send your one Qanon friend with asthma to sit in a nearby bar with a dufflebag filled with dozens of phones. Later claim you were all there having beers at that time.
Actually, as soon as investigators notice that 2 or 3 of those arrested didn't have phones and that their phones were all located at the same place, the gig could be up. Though that takes a little police perseverance. And not as easy as bringing your personal tracking device with you and recording your illegal acts.
Another thing the police/FBI should be and no doubt are doing is checking the flight records of planes into DC in the last 3 days prior to the Trump led rally and insurrection. I'm sure plenty from the East Coast drive, but the ones form farther afield flew in. Certainly true of Ashli form San Diego and the Midland TX flower lady who engaged in no vandalism except kicking down Pelosi's office door and stealing her gavel and a laptop and...