Enrique Tarrio, a former leader of the Proud Boys, and four other members of the far-right group have been indicted on seditious conspiracy charges in connection with the Jan. 6, 2021, attack on the U.S. Capitol, marking an escalation in the Justice Department’s riot probe.

A grand jury on Monday returned the indictment against Mr. Tarrio and co-defendants Ethan Nordean, Zachary Rehl and Dominic Pezzola, charging them with two new counts. The men already had been indicted in March on conspiracy and other charges stemming from the attack, with prosecutors alleging they worked together to develop plans to storm the building that day.

Mr. Tarrio’s lawyer, Nayib Hassan, said: “Mr. Tarrio looks forward to being vindicated of these allegations.” Lawyers for the other men couldn’t immediately be reached for comment.

Mr. Tarrio, 38 years old, wasn’t in Washington that day, but prosecutors said he led the planning for Jan. 6 and continued to direct and encourage the Proud Boys before and during the attack and claimed credit for what happened on social media.