The FBI’s application for a wiretap on a Trump campaign adviser was riddled with errors, omissions and uncorroborated information, the Justice Department inspector general said in a scathing report released Monday.

Inspector General Michael E. Horowitz found that FBI agents working on the investigation into alleged ties between President Trump and Russia during the 2016 presidential campaign submitted exaggerated or incomplete information to the Foreign Intelligence Surveillance Court.

An FBI attorney doctored evidence against Carter Page, an adviser to the Trump campaign, to get the FISA warrant renewed in 2017, he determined.

All told, Mr. Horowitz found a total of 17 inaccuracies or omissions in the FBI’s application to monitor Mr. Page, which launched the Russia collusion investigation that dogged Mr. Trump well into his presidency.

There were “serious performance failures” by both agents running the investigation and top FBI officials supervising the probe, Mr. Horowitz said.

“We are deeply concerned that so many basic and fundamental errors were made by three separate, hand-picked investigative teams, on one of the most sensitive FBI investigations … and even though those involved with the investigation knew that their actions were likely to be subjected to close scrutiny,” he wrote.

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In multiple instances, the FBI withheld evidence that contradicted information it had told FISA judges, the report concluded.

Perhaps the most damning revelation was the FBI’s failure to corroborate allegations by former British spy Christopher Steele, whose salacious and unverified dossier was used to obtain the warrant.

FBI agents omitted information that ran counter to some of Mr. Steele’s most inflammatory accusations, the nearly 440-page report found.

Agents failed to tell the FISA court that one of Mr. Steele’s subsources offered details that raised “significant questions” about the reliability of his information, according to the report.

They also withheld information repudiating Mr. Steele’s claims. Among the information were representations from Mr. Steele’s associates that his work was subpar and represented “poor judgment,” the report said.

The FBI also failed to tell the court that publicly available information supported allegations that Mr. Steele leaked information about Mr. Page to a news outlet.

But it was more than omissions. The FBI also exaggerated the scope of Mr. Steele’s work with the bureau in a previous investigation.

“We identified multiple instances in which factual assertions relied upon in the first FISA application were inaccurate, incomplete, or unsupported by appropriate documentation based upon information the FBI had in its possession at the time the application was filed,” Mr. Horowitz wrote.

The FBI’s misconduct didn’t stop after the investigation was over. In 2017, an FBI lawyer doctored an email from an unidentified government agency that claimed Mr. Page was a “source” for them.

The lawyer, who is not identified in the report, changed the email to give the opposite impression.

Mr. Horowitz said he could not find any direct evidence that political bias played a role in the errors, but he did not offer an alternative explanation for the FBI’s bungling.

Mr. Horowitz said the FBI was justified in opening the probe into the Trump campaign because it is obligated to investigate federal crimes and national security threats.

That assertion immediately drew pushback from U.S. Attorney John Durham, who is running a parallel investigation into the origins of the Russia probe.

“Based on the evidence collected to date, and while our investigation is ongoing, last month we advised the Inspector General that we do not agree with some of the report’s conclusions as to predication and how the FBI case was opened,” Mr. Durham said in a statement.

The U.S. attorney for Connecticut, Mr. Durham was tasked by Attorney General William P. Barr this year to oversee the investigation.

Mr. Durham noted that the inspector general’s authority was limited to information within the Justice Department, while his investigation culled evidence from “other persons and entities both in the U.S. and outside of the U.S.”

The report’s findings will give plenty of ammunition for both sides on Capitol Hill. Mr. Horowitz is scheduled to testify about his report before the Senate Judiciary Committee on Wednesday.

The president said the report’s findings were “far worse than he thought possible.”

“They fabricated evidence and they lied to the courts, and they did all sorts of things to have it go their way,” Mr. Trump said. “We’re lucky we caught ‘em. I think I’m going to put this down as one of our great achievements. Because what we found and what we saw — never, ever should this happen again in our country.”

Mr. Barr said the FBI’s actions were “a clear abuse of the FISA process.”

“The Inspector General’s report now makes clear that the FBI launched an intrusive investigation of a U.S. presidential campaign on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken,” he said in a statement.

Senate Judiciary Committee Chairman Lindsey Graham, South Carolina Republican, said the FBI’s Russia probe morphed into “a criminal enterprise.”

“It became a criminal enterprise to defraud the FISA court and continue an operation against President Trump,” Mr. Graham said. “These are statements I don’t make lightly.”

Democrats leading the impeachment charge against Mr. Trump seized on Mr. Horowitz’s conclusion that the FBI was justified in opening the investigation.

Rep. Adam B. Schiff, California Democrat and chairman of the House Permanent Select Committee on Intelligence, which led the impeachment investigation, called for the president to end his “baseless attacks on patriotic public servants.”

“It may be too much to hope that this report will put to rest the conspiracy theories about the 2016 election — especially now when the president and his allies are eager to distract from mounting evidence of his abuse of power with respect to Ukraine,” Mr. Schiff said.

Other key Democrats in the impeachment effort, Judiciary Committee Chairman Jerrold L. Nadler of New York and Oversight and Reform Committee Chair Carolyn B. Maloney of New York, seized on Mr. Horowitz’s conclusion that the investigation was opened properly.

“The report affirms that the DOJ and FBI had an authorized purpose to conduct temporary surveillance,” they said in a joint statement.

Mr. Horowitz cleared top brass at the FBI and the Justice Department. He concluded that they were unaware of the errors made at the lower levels.

Among the officials he said did not know about the mistakes were former FBI Director James B. Comey and Obama-era Deputy Attorney General Sally Q. Yates.