Top leaders of the far-right Proud Boys group, including its national chairman, Enrique Tarrio, have been charged with seditious conspiracy for plotting to storm the US Capitol to obstruct the certification of Joe Biden’s election win over Donald Trump on 6 January 2021.

The move by federal prosecutors to charge Tarrio and four other Proud Boys leaders with seditious conspiracy – in addition to previous charges of obstructing a congressional proceeding – marks a major development in the criminal investigation into the Capitol attack.

In the 33-page indictment unsealed in Washington DC on Monday, the justice department said Tarrio and his co-defendants Joseph Biggs, Ethan Nordean, Zachary Rehl and Dominic Pezzola for months used encrypted messaging apps to stop Biden’s certification by force.

The new charges against the proud Boys leadership come days before the parallel congressional inquiry into the Capitol attack is scheduled to start televised hearings that are expected to examine, in part, Trump’s personal culpability in the events of January 6.

Seditious conspiracy, which is challenging to prove, requires federal prosecutors to show beyond a reasonable doubt that at least two people agreed to use force to overthrow the government or to interfere with the execution of a US law.

The new indictment is the latest involving seditious conspiracy, after the justice department filed identical charges earlier this year against top members of the far-right Oath Keepers militia group, including its founder Stewart Rhodes, over the Capitol attack.

A bipartisan US Senate report linked seven deaths to the attack on the Capitol, which failed to stop certification of Biden’s win. Trump was impeached for inciting an insurrection but acquitted when enough Senate Republicans stayed loyal.

In adding on the seditious conspiracy charges, the justice department appeared to indicate that it has learned new information in recent weeks about the Proud Boys’ plans ahead of 6 January as a result of several significant developments.

One of the Proud Boys, who was originally charged with Tarrio and the other co-defendants for obstructing a congressional proceeding, Charles Donoghue pleaded guilty in April and accepted a plea deal to cooperate with the criminal investigation into the group.

Meanwhile, though the indictment also identified unindicted co-conspirators – “Person 1” is understood to be Jeremy Bertino and “Person 2” is likely Aaron Whallon Wolkind – neither of those men have been charged. A third top Proud Boy leader, John Stewart, also remains uncharged.

The government said in the indictment that on 20 December 2020, Tarrio created a chat called “MOSD Leaders Group” – described by Tarrio as a “national rally planning committee” – that included Nordean, Biggs, Rehl and other individuals who were not identified.

Through the rest of December, the government said, Proud Boys leaders used additional MOSD group chats to plan a “DC trip” and communicate to group members that they should go to the capital not wearing their familiar black and yellow colours but travel “incognito” instead.

The government said in the indictment that on 30 and 31 December 2020, Tarrio communicated with an individual – whose identity is known only to a grand jury – who sent him a nine-page document, called “1776 returns” in reference to the year of American independence from Britain. It laid out a plan to occupy “crucial buildings” on 6 January.

The document broadly outlined a plan to reconnoiter and storm crucial government buildings in Washington DC on 6 January, though not the Capitol itself, the New York Times earlier reported.

Tarrio is said to have received the “1776 returns” document from one of his girlfriends, who compared the plan to storming the Winter palace in St Petersburg that sparked the Russian Revolution in 1917, the New York Times reported.

The indictment cited a reference to that moment in the new indictment, drawing upon what appeared to be newly-uncovered text messages. After the Capitol attack ended, Bertino messaged Tarrio, “1776,” to which Tarrio responded: “The Winter Palace.”

Three days before the Capitol attack, a Proud Boy referred to only as “Person-3” posted a voice message in the MOSD Leaders Group that stated the “main operating theater should be out in front of the House of Representatives”, according to the indictment.

“That’s where the vote is taking place with all of the objections,” the person said, according to the indictment. “Plan the operations based around the front entrance to the Capitol building. I strongly recommend you use the National Mall and not Pennsylvania Avenue.”

The new pieces of evidence in the latest indictment were messages Bertino sent to Tarrio after the attack. “You know we made this happen,” Bertino said. Referring to the implications of obstructing Biden’s win, Bertino added: “They HAVE to certify today. Or it’s invalid.”

Tarrio was not in Washington DC on 6 January 2021, having been ordered to leave the capital by a judge after being arrested the day before for burning a Black Lives Matter banner at a church during a pro-Trump rally in December.

But the justice department has said that even though Tarrio was not accused of “physically taking part in the breach of the Capitol”, he “led the advance planning and remained in contact with other members of the Proud Boys during” the attack.

Eleven members of the Oath Keepers militia are also charged with seditious conspiracy.

Lawyers for Tarrio and the other four Proud Boys leaders have said there is no evidence they conspired to storm the Capitol, and that the MOSD group chats and the acquisition of tactical gear before 6 January were measures to protect themselves in case of potential altercations.