More than two dozen FBI informants were in Washington DC ahead of the riot at the US Capitol on 6 January 2021, but no full-time undercover agents were present or took part in the riot itself, according to a new justice department report.
The report said none of the agency's informants were authorised to enter the Capitol or join the riot, but four did enter the building.
The report also found that the FBI failed in the "basic step" of adequately using its field offices across the US to gather intelligence that could have predicted the riot.
Some on the right, including House Republicans, have for years promoted a fringe conspiracy theory that the FBI helped to orchestrate the riot.
On the day of the riot, a mob stormed the Capitol as the 2020 presidential election was being certified.
Hundreds have since been arrested and criminally charged for their participation.
President-elect Donald Trump has vowed to pardon "most" of the convicted 6 January rioters as soon as he returns to the White House in January.
- 'Man of his word': Jan 6 rioters expect Trump will keep pardon promise
The report, from the justice department's Office of the Inspector General, found that 26 "confidential human sources" - or paid informants - were in Washington on the day of the riot.
Three of them had been tasked with gathering information for domestic terrorism cases who might have been going to the rallies on 6 January, one of whom entered the Capitol building.
The remaining 23 had not been directed to be in the area and did so on their own initiative.
At the time, some were in contact or travelling with members of far-right groups including the Proud Boys and Oath Keepers.
Of the 26 in total, four confidential sources entered the Capitol during the riot. Another 13 entered the restricted area around the Capitol - a security perimeter established in preparation for election certification on 6 January.
None of the confidential sources who entered the Capitol or its environs were among those criminally charged with trespassing.
Confidential sources are different from full-time, trained undercover agents.
The justice department defines these sources as those "believed to be providing useful and credible information to the FBI" who warrant confidential handling.
In the wake of the riot, the FBI came under scrutiny from lawmakers who questioned the use of informants and whether more could have been done to gather intelligence and prevent the riot from taking place.
While the report compiled by justice department inspector Michael Horowitz determined that the FBI had identified a potential for violence that day and took "appropriate" steps, it missed a "basic step" by not canvassing its field officers for potential intelligence.
This step, in turn, "could have helped the FBI and its law enforcement partners with their preparations" ahead of the riot.
Approximately 1,572 people have been charged criminally in federal court for their participation in the riot.
The figure includes nearly 600 charged with assaulting, resisting or impeding law enforcement agents, and 171 charged with using a deadly or dangerous weapon.
Trump - who has referred to the rioters as "patriots" and "political prisoners" - said in a recent interview that he is "going to be acting very quickly" to pardon "most" of the people involved in the riot.
Despite the president-elect's promise, the justice department has continued to arrest and charge alleged participants in the riot in recent weeks.
