The leader of the Oath Keepers militia group, who was indicted Thursday on a series of charges including seditious conspiracy in connection with the Jan. 6 assault on the U.S. Capitol, made his first appearance before a judge Friday in a federal courtroom in Texas.

Stewart Rhodes, a former Army paratrooper and graduate of Yale Law School, could spend decades behind bars if convicted on all five federal counts he faces — including the most serious seditious conspiracy charge, which carries a maximum sentence of 20 years in prison.

A lawyer for Rhodes told ABC News Friday that the allegations against Rhodes were “lies,” and said that no members of the Oath Keepers ever “planned or conspired to attack the Capitol.”

In his Friday court appearance, Rhodes responded “Yes” when asked by Magistrate Judge Kimberly Priest Johnson if he understood the charges against him. He then waived his right to have the full indictment read aloud.

Prosecutors asked that Rhodes be detained while he is awaiting trial, and the judge set a detention hearing for Jan. 20. Rhodes will remain in custody until then.

The indictment of Rhodes, along with 10 other alleged members of the Oath Keepers, signals a significant escalation in the Justice Department’s sprawling investigation of the Jan. 6, 2021, attack and its prosecution of members of the far-right Oath Keepers militia group, described by prosecutors as a “large but loosely organized collection of individuals” who “explicitly focus on recruiting current and former military, law enforcement, and first-responder personnel.”

Prosecutors allege Rhodes and other Oath Keepers began coordinating as early as just after Election Day “to oppose by force the execution of the laws governing the transfer of presidential power” between outgoing President Donald Trump and incoming President Joe Biden, according to court papers.

While Rhodes himself is not alleged to have entered the Capitol during the attack, prosecutors say he did enter the restricted area surrounding the building and coordinated with Oath Keepers who were part of a military-style “stack” formation seen walking into the building up the east side steps. Prosecutors said in their indictment Thursday that the members of the so-called “stack” were specifically searching for House Speaker Nancy Pelosi, but left after they couldn’t find her.

In their 48-page indictment, investigators chronicled in detail Rhodes’ alleged communications with members of the group over private and encrypted apps, and their alleged accumulation of heavy weaponry and tactical gear that the group is accused of storing just outside Washington at a hotel in Virginia, where on Jan. 6 prosecutors say a so-called “Quick Reaction Force” of militia members waited on standby in case they were called into the city.

Nine of those charged in Thursday’s indictment had been previously charged in connection with the Jan. 6 attack as part of what was already the Justice Department’s largest and most complex conspiracy case tied to the insurrection.

In addition to Rhodes, 63-year-old Edward Vallejo of Arizona was arrested in Phoenix on Thursday and also charged with seditious conspiracy. Vallejo was allegedly part of the “Quick Reaction Force” that was lying in wait at the Virginia hotel.

After the riot, Rhodes and Vallejo allegedly met up at a restaurant where they “celebrated their attack” and discussed “next steps,” according to the indictment. Vallejo allegedly sent a message to a Signal chat group the morning after Jan. 6 where he discussed making a “recon” trip to the Capitol to probe the “defense line” put up by law enforcement in the wake of the attack, court papers said.

Vallejo also made his first appearance before a magistrate judge in Phoenix on Friday afternoon, where a public defender representing him said he plans to plead not guilty to all charges against him. The judge set a detention hearing for next Thursday as the Justice Department seeks to keep Vallejo behind bars pending further legal proceedings in his case.

The deployment of the rarely-used seditious conspiracy charge will pose a major test for the Justice Department in its investigation into the Capitol attack and the prosecution of Rhodes as the founder and self-described leader of the Oath Keepers.

Only days after the Jan. 6 attack, the then-acting U.S. attorney for Washington, D.C., Michael Sherwin, said prosecutors were considering the potential for seditious conspiracy charges against some of the most “heinous acts” that took place at the Capitol. But as the investigation crossed the one-year mark and the number of arrests stretched beyond 700, such charges had yet to materialize, with prosecutors instead opting to bring charges like conspiracy or obstruction of an official proceeding, which similarly carries a maximum sentence of 20 years in prison.

Attorney General Merrick Garland appeared to foreshadow Thursday’s charges last week in a speech marking the one-year anniversary of Jan. 6, when he addressed criticism of the department’s handling of the investigation and the lack of charges to date against the more prominent figures believed to have coordinated the assault on Congress.

“The actions we have taken thus far will not be our last,” Garland said. “The Justice Department remains committed to holding all Jan. 6 perpetrators, at any level, accountable under law — whether they were present that day or were otherwise criminally responsible for the assault on our democracy.”

John Sandweg, a former acting general counsel at the Department of Homeland Security, told ABC News that Thursday’s indictment “confirms that the attack on the Capitol was not just an impulsive act, but was part of a premeditated conspiracy to forcibly steal the levers of power.”

“It also demonstrates that, while much of the focus has been on the prosecution of those on lesser charges related to storming the Capitol, DOJ has been actively investigating the root causes of the attack,” he said. “The question remains how far up the food chain will the rest of the investigation lead, but this indictment significantly ups the ante.”

ABC News’ Juan Renteria, James Scholz and Mireya Villarreal contributed to this report.