Of the 125 federal arrests made so far in connection with the Jan. 6 riot at the Capitol, most have been relatively simple: Agents and prosecutors have put together cases largely by scouring the news and social media for incriminating photographs and videos, with some of the evidence almost comically easy to obtain.
But the inquiry into the Capitol assault, a huge effort that has focused its attention on as many as 400 people, took an important turn this week as prosecutors filed their first serious conspiracy charges, accusing three members of the right-wing militia group the Oath Keepers of plotting the incursion in advance. If, as they have promised, investigators are hoping to narrow their gaze on organized extremists who may have planned the attack, they are going to have to use a different and more difficult-to-master set of skills.
The F.B.I.’s most challenging work, legal scholars say, may have only just begun.
“It’s a lot harder to charge a conspiracy, especially compared to the first wave of cases where you basically had people confessing on video to federal crimes,” said Aitan Goelman, a former federal prosecutor who helped try Timothy McVeigh, the Oklahoma City bomber.
In making more conspiracy cases, the first question investigators must confront is how much conspiring actually went into the storming of the Capitol on Jan. 6. Five people died in the violent attack, and the final certification of President Biden’s election was abruptly interrupted as lawmakers fled the House and Senate floors.
Chilling videos and photos have emerged showing some people moving inside the building in tight formation, wearing military gear, carrying restraints and sometimes using hand signals or radios to communicate.
But many people appear to have acted spontaneously and, at least so far, have been accused of misdemeanors like unlawful entry and disorderly conduct.
The Oath Keeper case could be a model moving forward for more complicated cases. The criminal complaint shows investigators employing a variety of techniques in tracking down and charging the defendants: Thomas E. Caldwell, Donovan Crowl and Jessica Watkins. Mr. Caldwell said he intended to fight the charges at a hearing this week. Mr. Crowl and Ms. Watkins have not yet appeared in court to respond to the complaint.
Agents in their case pored through video footage at the Capitol looking for badges or insignia suggesting that the three accused militia members were part of the same group. They trolled social media accounts on platforms like Parler for any indications that the three were not only at the building, but had planned in advance to be there. And they obtained audio recordings of Ms. Watkins talking with others who are suspected of being Oath Keepers on Zello, a push-to-talk cellphone app that operates like a walkie-talkie.
Easy charges were brought early in the inquiry in an effort to get people into custody while investigations pressed forward. Prosecutors have echoed that notion in court, indicating that they are considering more serious charges against some defendants who have already been charged.
Shortly after the riot, the prosecutor in charge of the overarching inquiry, Michael Sherwin, the acting U.S. attorney in Washington, announced that some people could face sedition charges, which are difficult to bring and rarely filed.
To prove a seditious conspiracy, prosecutors need to show that at least two people agreed to use force to overthrow government authority or delay the execution of a U.S. law, such as stopping Congress from certifying the results of the election. The charge is powerful, carrying a maximum sentence of 20 years in prison.
But some prior sedition cases have fallen apart because prosecutors failed to prove that the defendants had a concrete plan to commit a physical attack, even if there was evidence of openly discussing bringing down the government. That defense could be more challenging in the Capitol riot cases, former federal prosecutors say, because the attack has already happened.
Adam Goldman, Katie Benner and Rebecca R. Ruiz contributed reporting.