When the Justice Department's inspector general, Michael E. Horowitz, rejected one of Mr. Trump's main attacks on the F.B.I. and declared the bureau had adequate reason to investigate the Trump campaign's ties to Russia, Mr. Barr again stepped in, issuing a statement saying that the F.B.I. instead should not have opened the investigation in 2016.

"The F.B.I. 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 both cases, Mr. Barr highlighted his belief that law enforcement officials overstepped their authority when they decided to investigate Trump campaign aides and that the president ultimately did nothing wrong. Mr. Barr long suspected that was the case before he became attorney general, and his time in office has crystallized his belief, friends and associates say.

Mr. Barr's willingness to side with Mr. Trump over law enforcement, even when it contradicts his own department's assessments, illustrates why he is one of Mr. Trump's most important allies.

His pronouncement on Monday comes at a time when the president is on the verge of being impeached over accusations that he abused his power. And Mr. Barr's past pronouncements that it is nearly impossible for the president to break the law when exercising his authorities have been adopted by Mr. Trump's allies as they try to push back on the impeachment inquiry.

John H. Durham, a federal prosecutor whom Mr. Barr appointed to run a separate criminal investigation into the origins of the Russia investigation, backed Mr. Barr's findings in his own highly unusual statement. "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 F.B.I. case was opened," Mr. Durham said.

He also noted that he had access to more information than Mr. Horowitz did, an apparent reference to Mr. Durham's trips to meet with foreign intelligence officials overseas and to his examination of any role the C.I.A. played in how the F.B.I. confronted Russia's election interference in 2016.

Mr. Barr said in the statement that the report by the inspector general "provided critical transparency and accountability."

The statements from the Justice Department's top official and one of his key investigators gave ammunition to Mr. Trump's supporters to dispute a major finding in the long-awaited report by Mr. Horowitz that was one of the few bright spots for the F.B.I. But it drew criticism from the F.B.I.'s supporters.

"I don't understand how Barr can on the one hand lavish praise on the inspector general and his team of lawyers and investigators, and in the next breath seek to undermine its key finding that the investigation was authorized and validly predicated," said Michael Bromwich, a former Justice Department inspector general who is now a lawyer in private practice. Mr. Horowitz has harshly criticized one of his clients, the former F.B.I. deputy director Andrew G. McCabe, for violations of the Justice Department's media policy.

"They either got it right or they didn't," Mr. Bromwich said.

Mr. Horowitz found that the F.B.I. had adequate reason in 2016 to lawfully open a full investigation into whether Trump campaign associates were wittingly or unwittingly helping Russia to interfere in the election. Called Crossfire Hurricane, the investigation eventually focused on four Trump campaign associates, two of whom were already the subjects of continuing Justice Department investigations.

The F.B.I. opened the inquiry amid a rash of leaked emails that had been stolen from the Democratic National Committee, and days after receiving intelligence that a Trump campaign adviser at the time, George Papadopoulos, had told Australian diplomats that a Russian had intermediary had offered information that could damage Hillary Clinton's campaign.

Only the tip from the Australian government was officially used "to predicate the opening of the Crossfire Hurricane investigation," Mr. Horowitz said and that was enough information to open the inquiry. His conclusion undercut accusations by the president and his allies that F.B.I. officials had carried out a politicized conspiracy to sabotage his campaign.

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