WASHINGTON – In the weeks following the deadly January 6 riot at the Capitol, federal prosecutors in Washington drew up a comprehensive plan to eradicate possible conspirators against the attackers and investigate them for links to the attack.

Prosecutors suggested that these lists could help organizers of the rally where President Donald J. Trump spoke just before the attack, anyone who helped pay the rioters to travel to Washington, and any member of the far-right groups that in the US include crowd that day.

Two of the prosecutors – trial lawyers who led the riot investigation – presented the plan to the FBI in late February, along with a roughly 25-page document setting out the strategy for uncovering possible conspiracies between the attackers and other people behind on condition of anonymity spoke to discuss an active investigation.

The aggressive plan was in line with the Justice Department’s public vow to indict those involved in the Capitol attack. But FBI officials flinched, citing concerns that the plan appeared to suggest investigating people with no evidence to suggest they committed crimes, and that doing so would be against the bureau’s policies and protection of the first amendment. It is not illegal to join any organization, including extremist groups, or to participate in protests or to fund travel to a rally.

FBI officials voiced their concern to officials at the Chief Justice Department in Washington, who eventually overturned the plan.

However, the decision by senior FBI and Justice Department officials to override the task force prosecutors came at a crucial time for the high-profile, far-reaching investigation, as the public and officials of the Biden government are accountable for the insurrection and called for a push to combat domestic extremism.

Justice Department and FBI spokesmen declined to comment.

The proposal also demonstrates the balancing act that newly sustained Justice Department leaders face as they attempt to counter domestic extremism and prevent terrorism without violating American civil liberties. The FBI was previously criticized for its response to the September 11, 2001 terrorist attacks, the aspects of which were condemned as an attack on civil liberties, and for its Cointelpro campaign in the 1950s and 1960s to spy on civil rights leaders and others.

Attorney General Merrick B. Garland said last week that even as he led the investigation into the 1995 Oklahoma City bombing during a previous stint at the Justice Department, investigators knew they needed to see to it that Americans’ civil liberties were protected.

“We promised to find the perpetrators, bring them to justice and do so in a way that respects the constitution,” Garland said.

FBI officials have emphasized the bureau’s efforts to stay within its boundaries when investigating protected activity. While preventing terrorism is a priority in the United States, “an investigation cannot be initiated solely on the basis of activities protected by the first amendment,” said Michael McGarrity, then head of the FBI’s counter-terrorism division, in the year 2019 in a statement from the house.

The office relies in large part on its large network of informants who provide tips and information on how to start an investigation, said current and former members of the FBI’s Joint Terrorism Task Force. But agents cannot investigate people simply because they are members of groups that advocate violent, racist, or anti-government ideologies.

Washington prosecutors encountered this restriction while trying to identify and track down individuals who participated in the January 6 attack. They also investigated whether the attack was more than a spontaneous riot that broke out after an emotionally charged rally, limited by Mr Trump’s admonitions to his supporters to contest Congressional certification that afternoon of the election.

In February, some prosecutors expressed frustration at being obstructed by senior Justice Department officials overseeing the investigation in the weeks leading up to the swearing-in of Mr. Garland and other Biden officials.

Prosecutors wanted to know more about who had spoken to Stewart Rhodes, leader of the Oath Keepers, a militia whose members had played a prominent role in conspiracy cases charged by the government in connection with the attack.

In a message posted on the Oath Keepers website, Mr Rhodes had urged members to come to Washington and stand up for Mr Trump. He was also part of an operation to provide security to Mr. Trump’s close associates, including Roger J. Stone Jr., who spoke at the rally that day.

Prosecutors wanted a search warrant for Mr. Rhodes. Militias like the Oath Keepers and right-wing nationalist groups like the Proud Boys had for years managed to largely evade FBI control as their protests and other public activities remained within the law.

But with members of such groups in the Capitol on January 6, some prosecutors expressed the hope that they now had reason to investigate their staff and leaders.

However, the law does not prohibit pressuring people to take part in a protest or support a politician, even if the statements are provocative. and investigators found no evidence that Mr Rhodes had helped arrange anything more than bodyguards for the speakers.

Justice Department officials, including Michael R. Sherwin, an officer who was overseeing the January 6 investigation at the time, denied prosecutors’ request for a search warrant on Mr. Rhodes, according to two people who were briefed on the deliberations . They concluded that the prosecutors lacked a likely cause for doing so without violating his civil liberties and rights.

Following the dispute, two of the lead task force prosecutors contacted the FBI’s Terrorism Operations Department to inform investigators of their proposed strategy to review the insurgency. They suggested that investigators look at rally organizers and organizations such as militia groups.

Among the FBI officers who opposed the approach, according to those informed about the plan, was Deputy Director Paul M. Abbate. After office officials discussed the presentation with Justice Department officials, the assistant attorney general’s chiefs – including Matthew S. Axelrod, then the second-largest officer in the office – briefed Channing D. Phillips, the acting U.S. attorney in Washington, on the Prosecutors would not take such an approach to the investigation.

The investigation, which continues to be led by federal attorneys and FBI agents in Washington, has led to the arrest of over 400 defendants in at least 45 states. About 30 were charged with more serious crimes, including conspiracy, according to the Justice Department.