Though federal officials have arrested more than 570 alleged participants, the FBI at this point believes the violence was not centrally coordinated by far-right groups or prominent supporters of then-President Donald Trump, according to the sources, who have been either directly involved in or briefed regularly on the wide-ranging investigations.
"Ninety to ninety-five percent of these are one-off cases," said a former senior law enforcement official with knowledge of the investigation. "Then you have five percent, maybe, of these militia groups that were more closely organized. But there was no grand scheme with Roger Stone and Alex Jones and all of these people to storm the Capitol and take hostages."
Stone, a veteran Republican operative and self-described "dirty trickster", and Jones, founder of a conspiracy-driven radio show and webcast, are both allies of Trump and had been involved in pro-Trump events in Washington on Jan. 5, the day before the riot.
FBI investigators did find that cells of protesters, including followers of the far-right Oath Keepers and Proud Boys groups, had aimed to break into the Capitol. But they found no evidence that the groups had serious plans about what to do if they made it inside, the sources said.
Prosecutors have filed conspiracy charges against 40 of those defendants, alleging that they engaged in some degree of planning before the attack.
They alleged that one Proud Boy leader recruited members and urged them to stockpile bulletproof vests and other military-style equipment in the weeks before the attack and on Jan. 6 sent members forward with a plan to split into groups and make multiple entries to the Capitol.
But so far prosecutors have steered clear of more serious, politically-loaded charges that the sources said had been initially discussed by prosecutors, such as seditious conspiracy or racketeering.
The FBI's assessment could prove relevant for a congressional investigation that also aims to determine how that day's events were organized and by whom.
Senior lawmakers have been briefed in detail on the results of the FBI's investigation so far and find them credible, a Democratic congressional source said.
The chaos on Jan. 6 erupted as the U.S. Senate and House of Representatives met to certify Joe Biden's victory in November's presidential election.
It was the most violent attack on the Capitol since the War of 1812, forcing lawmakers and Trump's own vice president, Mike Pence, to scramble for safety.
Four people died and another died the following day, and more than 100 police officers were injured.
Trump made an incendiary speech at a nearby rally shortly before the riot, repeating false claims that the 2020 election was stolen and urging supporters to march on the Capitol to pressure lawmakers to reject Biden's victory.
In public comments last month to the Democratic-led congressional committee formed to investigate the violence, police officers injured in the mayhem urged lawmakers to determine whether Trump helped instigate it. Some Democrats have said they want him to testify.
But the FBI has so far found no evidence that he or people directly around him were involved in organizing the violence, according to the four current and former law enforcement officials.
More than 170 people have been charged so far with assaulting or impeding a police officer, according to the Justice Department. That carries a maximum sentence of 20 years.
But one source said there has been little, if any, recent discussion by senior Justice Department officials of filing charges such as "seditious conspiracy" to accuse defendants of trying to overthrow the government. They have also opted not to bring racketeering charges, often used against organized criminal gangs.
Senior officials had discussed filing such charges in the weeks after the attack, the sources said.
Prosecutors have also not brought any charges alleging that any individual or group played a central role in organizing or leading the riot. Law-enforcement sources told Reuters no such charges appeared to be pending.
Conspiracy charges that have been filed allege that defendants discussed their plans in the weeks before the attack and worked together on the day itself. But prosecutors have not alleged that this activity was part of a broader plot.
Some federal judges and legal experts have questioned whether the Justice Department is letting defendants off too lightly.
Judge Beryl Howell in July asked prosecutors to explain why one defendant was allowed to plead to a misdemeanor charge carrying a maximum sentence of six months, rather than a more serious felony charge.
Spokespeople for the Justice Department and U.S. Attorney's office in Washington, which is leading the Jan. 6 prosecutions, declined to comment.
The congressional committee investigating the attack will talk with the FBI and other agencies as part of its probe.