A trial that started Monday in Washington, D.

Prosecutors are saying at the opening of the most serious case to reach trial in the attack on the U.S. Capitol that the founder of the Oath Keepers extremist group and four associates planned for an “armed rebellion” to stop the transfer of presidential power.

The founder of the Oath Keepers and four associates are on trial in the U.S. Capitol attack on charges that include seditious conspiracy — a rare Civil War-era charge that strikes to the heart of what prosecutors say happened that day.

Sedition and treason cases have been rare in U.S. history.