WASHINGTON — The FBI should have done more to gather intelligence before the Capitol riot, according to a watchdog report Thursday that also said no undercover FBI employees were on the scene Jan. 6, 2021, and none of the bureau's informants was authorized to participate.

The report from the Justice Department inspector general's office knocks down a fringe conspiracy theory advanced by some Republicans in Congress that the FBI played a role in instigating the events that day, when rioters determined to overturn Republican Donald Trump's 2020 election loss to Democrat Joe Biden stormed the building in a violent clash with police.

The review, released nearly four years after a dark chapter in history that shook the bedrock of American democracy, was narrow in scope, but aimed to shed light on gnawing questions that have dominated public discourse, including whether major intelligence failures preceded the riot and whether the FBI in some way provoked the violence. It's the latest major investigation about a day unlike any other in U.S. history, one that has already yielded congressional inquiries and federal and state indictments.

The report offers a mixed assessment of the FBI's performance in the run-up to the riot, crediting the bureau for preparing for the possibility of violence and for trying to identify known "domestic terrorism subjects" who planned to come to Washington that day.

But it said the FBI, in an action the now-deputy director described as a "basic step that was missed," failed to canvas informants across all 56 of its field offices for any relevant intelligence. That was a step, the report concluded, "that could have helped the FBI and its law enforcement partners with their preparations in advance of January 6."

In addition, the watchdog found that 26 FBI informants were in Washington for election-related protests on Jan. 6, and though four entered the Capitol during the riot, none had been authorized to do so by the bureau or to break the law or encourage others to do so. Many of the 26 informants did provide the FBI with information before the riot, but it "was no more specific than, and was consistent with, other sources of information" that the FBI had acquired from other sources.

The FBI said in a letter responding to the report that it accepts the inspection general's recommendation "regarding potential process improvements for future events."

The lengthy review was launched days after the riot as the FBI faced questions over whether it had missed warnings signs or had adequately disseminated intelligence it had received, including a Jan. 5, 2021, bulletin prepared by the FBI's Norfolk, Virginia, field office that warned of the potential for "war" at the Capitol. The inspector general found that the information in that bulletin was broadly shared.

FBI Director Chris Wray, who announced this week his plans to resign at the end President Joe Biden's term in January, has defended his agency's handing of the intelligence report. He told lawmakers in 2021 that the report was disseminated though the joint terrorism task force, discussed at a command post in Washington and posted on an internet portal available to other law enforcement agencies.

"We did communicate that information in a timely fashion to the Capitol Police and (Metropolitan Police Department) in not one, not two, but three different ways," Wray said at the time.

Separately, the report said the FBI's New Orleans field office was told by a source between November 2020 and early January 2021 that protesters were planning to station a "quick reaction force" in northern Virginia "in order to be armed and prepared to respond to violence that day in DC, if necessary."

That information was shared with the FBI's Washington Field Office, members of intelligence agencies and some federal law enforcement agencies the day before the riot, the inspector general found. But there was no indication the FBI told northern Virginia police about the information, the report said. An FBI official told the inspector general there was "nothing actionable or immediately concerning about it."

A cache of weapons at a Virginia hotel as part of a "quick reaction force" was a central piece of the Justice Department's seditious conspiracy case against Oath Keeper leader Stewart Rhodes and other members of the far-right extremist group.

The conspiracy theory that federal law enforcement officers entrapped members of the mob has been spread in conservative circles, including by some Republican lawmakers. Rep. Clay Higgins, R-La., recently suggested on a podcast that agents pretending to be Trump supporters were responsible for instigating the violence.

And former Rep. Matt Gaetz, R-Fla., who withdrew as Trump's pick as attorney general amid scrutiny over sex trafficking allegations, sent a letter to Wray in 2021 asking how many undercover agents or informants were at the Capitol on Jan. 6 and if they were "merely passive informants or active instigators."

It wasn't previously clear how many FBI informants were in the crowd that day. Wray refused to say during a congressional hearing last year how many of the people who entered the Capitol and surrounding area on Jan. 6 were either FBI employees or people with whom the FBI had made contact. But Wray said the "notion that somehow the violence at the Capitol on January 6 was part of some operation orchestrated by FBI sources and agents is ludicrous."

One FBI informant testified last year at the trial of former Proud Boys leader Enrique Tarrio about marching to the Capitol with his fellow extremist group members, and described communicating with his handler as the mob of Trump supporters swarmed the building. But the informant wasn't in any of the Telegram chats the Proud Boys were accused of using to plot violence in the days leading up to Jan. 6.