Stewart Rhodes, the leader and founder of the far-right Oath Keepers militia, was arrested on Thursday in connection with the Jan. 6 attack on the Capitol, his lawyer said

The lawyer, Jonathan Moseley, said he was not yet aware of the charges Mr. Rhodes was facing, adding that F.B.I. agents took him into custody on Thursday afternoon.

The arrest of Mr. Rhodes was a major step forward in the sprawling investigation of the Capitol attack..

Mr. Rhodes, a former Army paratrooper who went on to earn a law degree at Yale, has been under investigation for his role in Jan. 6 since at least last spring when, against the advice of his lawyer, he sat down with F.B.I. agents for an interview in Texas. He was at the Capitol on Jan. 6, communicating by cellphone and a chat app with members of his team, many of whom went into the building. But there is no evidence that he entered the Capitol.

The Oath Keepers, along with the Proud Boys, have emerged as the most prominent far-right extremists to have taken part in the assault on the Capitol. Prosecutors have collected reams of evidence against them, including encrypted cellphone chats and recordings of online meetings. They have charged its members not only with forcing their way into the building in a military-style “stack,” but also with stationing an armed “quick reaction force” at a hotel in Virginia to be ready to rush into Washington if needed.

Through their lawyers, members of the Oath Keepers who are already facing charges have said that they converged on Washington just before Jan. 6 as part of a security detail hired to protect conservative celebrities like Roger J. Stone, Jr., the longtime ally of former President Donald J. Trump.

In an interview with The New York Times this summer, Mr. Rhodes expressed frustration that several members of his group had “gone off mission” by entering the Capitol on Jan. 6, quickly adding, “There were zero instructions from me or leadership to do so.”

But at least four Oath Keepers who were at the Capitol that day and are cooperating with the government have sworn in court papers that the group intended to breach the building with the goal of obstructing the final certification of the Electoral College vote.

Mr. Rhodes has also attracted the attention of the House select committee investigating Jan. 6, which issued him a subpoena in November. In a letter at the time, House investigators noted that Mr. Rhodes had taken part in several events designed to question the 2020 presidential election throughout that fall and winter.

On Election Day, the letter said, Mr. Rhodes said that an “honest” count of the votes could only result in a victory for Mr. Trump and called on members of his group to “stock up on ammo” and prepare for a “full-on war in the streets.”

Within a week of Election Day, Mr. Rhodes had told the conspiracy theorist Alex Jones that he had men stationed outside Washington prepared to act at Mr. Trump’s command.

Around the same time, federal prosecutors say, he urged his fellow Oath Keepers at an online meeting to support Mr. Trump, calling him the “duly elected president” and adding: “You can call it an insurrection or you can call it a war or fight.”

The drumbeat continued through the winter, prosecutors say, as Mr. Rhodes appeared at a pro-Trump rally in Washington on Dec. 12, 2020, and called on Mr. Trump to invoke the Insurrection Act, suggesting that a failure to do so would result in a “much more bloody war.” At the rally, Mr. Rhodes acknowledged in a television interview that he and members of his group were there to provide security for celebrity speakers along with another shadowy paramilitary organization, the First Amendment Praetorian.

On Jan. 4, just two days, before the storming of the Capitol, Mr. Rhodes posted an article on the Oath Keepers website calling on “all patriots” to “stand tall in support of President Trump’s fight to defeat the enemies foreign and domestic who are attempting a coup.”

With his distinctive black eye patch — the result of a gun accident — Mr. Rhodes has been a fixture on the far right almost from the day in 2009 that he announced the creation of the Oath Keepers at a rally in Lexington, Mass., the site of a famous Revolutionary War battle.

At the event, Mr. Rhodes laid out an antigovernment platform for the current and former law enforcement and military personnel who joined his group, saying that his plan was for members to disobey certain illegal orders from officials and instead to uphold their oath to the Constitution.

During the years that President Barack Obama was in office, the Oath Keepers repeatedly inserted themselves into prominent public conflicts, often playing the role of heavily armed vigilantes. In 2014, for instance, they turned up at a cattle ranch in Nevada after its owner, Cliven Bundy, engaged in an armed standoff with federal land management officials. That same year, members of the group went to Ferguson, Mo. on a self-appointed mission to protect local businesses from riots prompted by the death of Michael Brown, a Black man who shot by the police.

But after Mr. Trump’s took office, Mr. Rhodes and the Oath Keepers pivoted away from their antigovernment views and appeared to embrace the new spirit of nationalism and suspicions of a deep-state conspiracy that had taken root in Washington. Like other far-right groups such as the Proud Boys, the Oath Keepers also opposed — often physically — the Black Lives Matter protests that erupted in the wake of the murder of George Floyd by the police in Minneapolis.

In late March, Mr. Rhodes himself publicly acknowledged that the F.B.I. was after him, declaring during a fiery speech on the Texas border that the Justice Department had undertaken a “persecution campaign” against his group.

“I may go to jail soon, not for anything I actually did, but for made-up crimes,” he said.