WASHINGTON - The U.S. Justice Department's inspector general says FBI leaders were justified in opening an inquiry into alleged ties between the Trump presidential campaign and Russian operatives, saying investigators found no evidence that the probe was politically motivated.

But the independent inspector general, Michael Horowitz, also sharply criticized the FBI in his report released Monday for its handling of the court authorized surveillance of a former Trump campaign advisor, during its probe of Russian meddling in the 2016 U.S. presidential election.

The report amounts to a rejection of Trump's repeated claim that the FBI investigation, dubbed Crossfire Hurricane, was carried out in an effort to undo his presidency. The probe was later taken over by special counsel Robert Mueller.

The FBI "had an authorized purpose when it opened Crossfire Hurricane to obtain information about, or protect against, a national security threat or federal crime, even though the investigation also had the potential to impact constitutionally protected activity," Horowitz wrote in the 400-plus-page report.

While Horowitz said he had found no evidence that FBI investigators were influenced by anti-Trump animus, he wrote that investigators had uncovered "serious performance failures by the supervisory and non-supervisory agents with responsibility over the FISA applications" in its surveillance of former Trump campaign aide Carter page.

"That so many basic and fundamental errors were made by three separate, hand-picked teams on one of the most sensitive FBI investigations that was briefed to the highest levels within the FBI, and that FBI officials expected would eventually be subjected to close scrutiny, raised significant questions regarding the FBI chain of command's management and supervision of the FISA process," the report states.

The FBI began looking into possible ties between the Trump campaign and Russia in July 2016 after receiving a tip that then Trump foreign policy advisor George Papadopoulos had claimed that the Russian government was willing to help the Trump campaign by releasing damaging information about Democrat Hillary Clinton in the form of thousands of emails stolen from the Democratic National Committee.

The bureau later obtained a court warrant to surveil Page after he had left the campaign. Page at the time was suspected of having ties to the Russian government and the surveillance continued into 2017.

In a statement, Attorney General William Barr said the report "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."

"In the rush to obtain and maintain FISA surveillance of Trump campaign associates, FBI officials misled the FISA court, omitted critical exculpatory facts from their filings, and suppressed or ignored information negative the reliability of their principal source," Barr wrote.