Unless they are looking for ONLY some type of govt trespass charge. They will have a hard time of convicting them of anything else based on just cell phone pings. I doubt they they will find it to be worth it. They could match some names to faces caught on security footage, but who knows how swiftly they will investigate. I think for now, they just want to make an example of the most obvious.
I think for now they will want to know where the F those people are right now and chat them up about inadvisability of "attending" the inauguration or any other events in DC any time soon, or for that matter any events at their respective state capitol buildings.
The FBI already said in a public statement that if you end up identified as having been there then you can expect a knock at the door after you get home. Not sure what "there" means to the FBI technically speaking but if they turn up a mobile number of someone in proximity to or association with someone who gained entry to the Capitol complex during the incursion, that probably sounds close enough to "there" to warrant a visit.
It's not like this was a demonstration that drifted to violence in the shopping district and some plate glass windows got smashed. This was an attack on the core of the United States government sitting under one roof. They were not flying a hijacked plane to finish off the known target list of September 11, 2001 by a foreign enemy. However, they may as well have been doing that for the harm some of Wednesday's marauders even stated they intended, and the rest either enabled or helped mask just by the numbers of their presence. Zip ties and weapons and explosives parked not far away are not the stuff of a peaceful demonstration and neither was the language of the guy who incited the crowd to go there and fight.
On pursuit of the widening circle: Think Timothy McVeigh. The government wanted to talk to a whole lot of people who even ever
met McVeigh or Nichols after the bombing in Oklahoma City. They interviewed 28,000 individuals. Michael Fortier had done far more than just meet McVeigh, although he refused to take part in preparations for the bombing... he had looked at the proposed site, and six months prior to the bombing, both Fortier and his wife had seen diagrams of the bomb McVeigh planned to build. Both testified at trial... it should give anyone pause to realize that Fortier got a 12-year
reduced sentence for "failing to warn the government" about what they knew of McVeigh's plans.