“I’m walking on sunshine,” the man, Kevin Mathewson, said the next day. A local private investigator and former city alderman, he had attended every day of the trial, in which he had more than a passing interest.

Mr. Mathewson had become a prominent and divisive figure in Kenosha. Days after George Floyd was killed by a police officer in Minneapolis, Mr. Mathewson had created an organization called the Kenosha Guard, an armed group that declared its intent in a Facebook post “to deter rioting/looting” amid racial justice demonstrations in Kenosha. In August 2020, after the police shooting of a Black man named Jacob Blake brought a wave of protests and rioting to the city, Mr. Mathewson had written on the Guard’s Facebook page urging Kenoshans to take to the streets with guns to defend the city. His Aug. 25 post went viral, drawing thousands of RSVPs and comments threatening violence.

Mr. Mathewson’s call to arms was one of several in Kenosha that day, which collectively brought dozens of mostly white armed paramilitaries into the streets of the city’s small downtown, creating a heavily armed confrontation with demonstrators that came to a head with the Rittenhouse shootings. Mr. Mathewson went home hours before the shootings, and no evidence ever connected Mr. Rittenhouse — whom Mr. Mathewson said he had never met — to his Facebook post. But his proximity to the incident led to him being banned from Facebook, where his Aug. 25 post had been flagged repeatedly for violating the platform’s ban on militia activity and had left an aura of suspicion around him.