On March 4, Judge Tanya Chutkan presided over jury selection for a criminal trial in a Jan. 6, 2021, case. That date had long been scheduled, though there was a different defendant seated before her than had been planned: In the place of former President Donald Trump sat a 57-year-old Pennsylvania man named Lynnwood Nester.

On Jan. 6, 2021, Mr. Nester had traveled to Washington with a group of fellow insurrectionists to attend the former president’s rally at the Ellipse, then walked, carrying a cane, toward the Capitol, where the group stopped to pose for photographs. He then proceeded inside through an opened door, wandered around the building for 10 minutes and left. He caused no damage and took nothing with him.

On his first day in court, Mr. Nester sat next to his attorney wearing a collared shirt and a tie, his glasses perched atop his gray hair. He glumly surveyed the potential jurors seated in the benches behind him, 12 of whom would later that week determine his fate. Mr. Trump, whose criminal trial has been put on hold pending Supreme Court review, spent that week on the campaign trail.

Mr. Nester is the among the latest in a long string of Jan. 6 insurrectionists who have over the past three and a half years sat for trial in the E. Barrett Prettyman U.S. Courthouse in Washington, D.C. Over the past several months, I have spent countless hours observing these trials: While early and high-profile cases attracted media attention, these days many unfold in nearly empty courtrooms. The corridors are hushed, the opposing attorneys collegial, the judges well acquainted with their routines.

Some of the trials have delivered long-delayed justice: The week before Mr. Nester appeared, a Kentuckian named Michael Sparks — the very first rioter to have broken into the Capitol building on Jan. 6 — was found guilty of all six charges. Members of the Oath Keepers and the Proud Boys have been sentenced to significant prison terms.

Yet for every violent rioter justly tried and punished, there have been many nonviolent offenders summoned to court. A few weeks ago, I watched as a 59-year-old woman stood weeping before a judge as she apologized for participating in the riot and pleaded guilty to two misdemeanors, her first criminal offenses.