A document found by federal prosecutors in the possession of a far-right leader contained a detailed plan to surveil and storm government buildings around the Capitol on Jan. 6 last year, people familiar with the document said on Monday.

The document, titled “1776 Returns,” was cited by prosecutors last week in charging the far-right leader, Enrique Tarrio, the former head of the Proud Boys extremist group, with conspiracy. The indictment of Mr. Tarrio described the document in general terms, but the people familiar with it added substantial new details about the scope and complexity of the plan it set out for directing an effort to occupy six House and Senate office buildings and the Supreme Court last Jan. 6.

The document does not specifically mention an attack on the Capitol building itself. But in targeting high-profile government buildings in the immediate area and in the detailed timeline it set out, the plan closely resembles what actually unfolded when the Capitol was stormed by a pro-Trump mob intent on disrupting congressional certification of President Biden’s Electoral College victory.

Many questions remain about the document, including who wrote it and how it made its way to Mr. Tarrio, according to prosecutors, on Dec. 30, 2020, as President Donald J. Trump was engaged in a series of overlapping schemes to keep himself in power. The people familiar with the document said other evidence the government has gathered suggests that it may have been provided to Mr. Tarrio by one of his girlfriends at the time.

Prosecutors have not accused Mr. Tarrio of using the document to guide the actions of the Proud Boys who played a central role in the Capitol attack. Nor do the charges against him offer any evidence that he shared the document with his five co-defendants: Ethan Nordean, Joseph Biggs, Zachary Rehl, Charles Donohoe and Dominic Pezzola.

But the document could help explain why prosecutors chose to charge Mr. Tarrio with conspiracy, even though he was not at the Capitol during the attack. And it appears to be the first time that prosecutors have sought to use evidence of a specific written plan to storm and occupy government buildings in their wide-ranging investigation into the attack and what led up to it.

Mr. Tarrio’s lawyer, Nayib Hassan, declined to comment.

Broken into five parts — Infiltrate, Execution, Distract, Occupy and Sit-In — the nine-page document recommends recruiting at least 50 people to enter each of the seven government buildings and advises protesters to appear “unsuspecting” and to “not look tactical,” the people familiar with it said.

After ensuring that crowds at the buildings are “full and ready to go,” the document suggests that “leads and seconds” should enter and open doors for others to go in, “causing trouble” to distract security guards, if necessary.

Should the crowds fail to gain entrance to the buildings quickly, the document suggests pulling fire alarms at nearby stores, hotels and museums to further distract guards or the police, the people said. It then says protesters should occupy the buildings and conduct sit-ins, even recommending slogans for people to chant, like “We the people” and “No Trump, No America.”

The document also makes suggestions for the days leading up to Jan. 6, the people said, advising protesters to “scope out” road closures near the seven target buildings. On the morning of the protest, they added, the document suggests having “scouts” drive around the buildings to look for “roadblocks.”

While much of document is marked for “internal” use, the people said, it contains a section, known as the “Patriot Plan,” that appears to have been meant for public distribution. This part of the document suggests that crowds begin to gather at the seven buildings at 1 p.m. on Jan. 6, the people familiar with it said. Thirty minutes later, the public part of the plan calls for the crowds to await a “signal from lead” and then to “storm” the buildings.

The indictment of Mr. Tarrio, who recently stepped down as chairman of the Proud Boys, was a significant development in the Justice Department’s sprawling investigation of the Capitol attack and the second time in recent months that charges had been brought against a leader of an extremist group that played a prominent role in the assault.

In January, prosecutors indicted Stewart Rhodes, the leader of the Oath Keepers militia, with seditious conspiracy for what the government has described as a plot to violently disrupt the work of Congress.

All together, more than 30 Proud Boys have been charged so far in connection with the attack, and many more took part in it but have not been charged. Members of the group, who have long served as vocal — and often violent — advocates for Mr. Trump, were instrumental in several key moments in the riot, including one at the Capitol’s security perimeter that resulted in the first breach of the barricades.

Prosecutors are still investigating the group. In court papers filed last week, they said they had recently carried out search warrants at the homes in North Carolina and central Pennsylvania of two top members of the group. Those members were later identified as Jeremy Bertino and John C. Stewart, according to people familiar with the matter.

Unlike many of his subordinates, Mr. Tarrio was not in Washington on Jan. 6, having been arrested two days earlier for vandalizing a Black Lives Matter banner at a Black church in the city after a pro-Trump rally in December 2020 that spiraled into violence after nightfall. Charged with a second crime — the possession of two high-capacity rifle magazines — he was ordered to leave Washington by a local judge as part of his release agreement.

But federal prosecutors said in their indictment that although Mr. Tarrio was not accused of “physically taking part in the breach of the Capitol,” he nonetheless “led the advance planning and remained in contact with other members of the Proud Boys during” the storming of the building.

Prosecutors have claimed, for example, that Mr. Tarrio issued orders before the attack for members of the group to leave behind their traditional black-and-yellow polo shirts and remain “incognito” when they arrived in Washington. In the days leading up to the attack, Mr. Tarrio also took part in a private Telegram group chat with other Proud Boys called the Ministry of Self-Defense and appeared to watch from a distance as several leaders and members of the group stormed the Capitol, prosecutors said.

Arrested last Tuesday, in his hometown, Miami, Mr. Tarrio is expected to appear in court there on Tuesday for a hearing to determine his bail. On Monday, prosecutors released court papers recommending that he remain in custody and arguing that he encouraged his compatriots to press on even as they broke into the Capitol.

“Proud Of My Boys and my country,” Mr. Tarrio posted on social media not long after lawmakers were chased from the building, delaying the final certification of the presidential election.

Around the same time, prosecutors said, a member of the Proud Boys posted a message in an encrypted chat asking, “Are we a militia yet?”

Mr. Tarrio responded with a one-word voice note, prosecutors said: “Yep.”