Washington (CNN) The Justice Department's inspector general took a big swing at the conspiracy theories peddled by President Donald Trump and others about the FBI's investigation into Russian meddling in the 2016 presidential election.

But the IG's much anticipated report released Monday also detailed errors the FBI committed during the investigation, specifically about surveillance warrants issued for a former Trump campaign aide.

Here are the key takeaways:

Legal and unbiased origin of the probe

In no uncertain words, the inspector general says there was no political conspiracy to undermine Trump's 2016 campaign.

Horowitz did not find "documentary or testimonial evidence that political bias or improper motivation influenced the decisions" to launch the investigation.

The report accuses a former FBI lawyer of altering a document related to the surveillance of campaign aide Carter Page. But Horowitz did not find that it undermined the overall validity of the surveillance.

Conspiracy theories debunked

The report essentially rebuts more than two years of talking points by Trump and Republicans about a deep-state effort to derail his campaign.

There were no FBI spies planted in Trump Tower, for instance. And the famed dossier by ex-British spy Christopher Steele was not the reason the investigation was launched, the IG report states.

Horowitz specifically says that Peter Strzok, a former senior counterintelligence officer, and Lisa Page, a former FBI attorney, whom Trump has repeatedly vilified and mocked in crude ways , did not act out of bias or unduly influence the start of the investigation.

The IG's office found that the FBI did not try to recruit members of the Trump campaign as informants, and did not to try infiltrate the campaign itself -- either by instructing sources to get hired onto the campaign, or by sending sources into campaign spaces to collect information.

Horowitz will be pressed to explain his conclusions Wednesday before an unfriendly audience: Trump ally Lindsey Graham's Senate Judiciary Committee.

Serious mistakes by the FBI

While the report doesn't back up conspiracy theories promoted by Trump and his allies, the inspector general details 17 "significant inaccuracies and omissions" in four applications for surveillance targeting Page.

A low-level FBI lawyer -- now under investigation but unnamed in the report-- doctored an email that he sent to a supervisory FBI agent as they tried to determine whether they needed to inform a special court overseeing surveillance warrants that Page had been a US government source in the past.

The information was not included in the surveillance warrant applications, but it could have been exculpatory because some of Page's past contacts with Russians were approved by the US government, the IG report said.

The inspector general also found highly partisan instant messages, insulting Trump and calling Vice President Mike Pence "stupid," sent by the low-level FBI lawyer. After the election, the FBI lawyer said in a message: "Viva le resistance."

While it's unclear how significant a role the altered document played in the FBI's investigation of Page, Horowitz did not find that it undermined the overall validity of the surveillance.

The FBI and Justice Department also misstated a key piece of information about Steele in the FISA applications, the IG found, saying that while Steele had previously provided information to the FBI that helped further criminal investigations, his reporting had never been used in a criminal proceeding.

In unusual move, Barr and Durham go on the attack

Attorney General William Barr and John Durham, the US attorney leading a separate review for Barr, are unhappy with the IG's conclusions about a lack of political bias.

In an unusual set of public statements Monday, Barr and Durham said the FBI investigation into Trump's campaign and Russia was flawed from the start.

"The inspector general's report now makes clear that the FBI launched an intrusive investigation of a US presidential campaign on the thinnest of suspicions, that, in my view, were insufficient to justify the steps taken," Barr said in a statement. "It is also clear that, from its inception the evidence produced by the investigation was consistently exculpatory."

Durham emphasized that he's doing a more extensive investigation into the FBI's Russia probe, promising there's more to come from the top levels of the Justice Department.

"Our investigation has included developing information from other persons and entities, both in the US and outside of the US," Durham said.