The FBI was justified in opening a probe into ties between Donald Trump's presidential campaign and Russia and did not act with political bias, despite "serious performance failures" up the bureau's chain of command, a highly anticipated report has said.
The findings of the report, by the Justice Department's inspector general General Michael Horowitz, undercut President Trump's claim that he has been the target of a "witch hunt".
The report insisted the bureau was justified in eavesdropping on a former adviser to Mr Trump.
Mr Horowitz also found that the FBI had a legal "authorised purpose" to ask for court approval to begin surveillance of Carter Page, a campaign adviser to Mr Trump.
However, he found a total of 17 "basic and fundamental" errors and omissions in the original application and all subsequent renewals to the Foreign Intelligence Surveillance Court (FISA).
The FBI investigation, opened in the summer of 2016 ahead of the November election pitting Mr Trump against Democratic candidate Hillary Clinton, was taken over in May 2017 by former FBI chief Robert Mueller after Mr Trump fired James Comey as the agency's director.
Mr Mueller's 22-month special counsel investigation detailed a Russian campaign of hacking and propaganda to sow discord in the United States, harm Ms Clinton and boost Mr Trump.
Former special counsel Robert Mueller. Source: AAP
Democrats have accused Mr Trump of seeking to discredit a legitimate investigation that detailed extensive interactions between his campaign and Russia.
But US Attorney John Durham, who is running a separate criminal investigation on the origins of the Russia probe, said in a prepared statement that "we do not agree with some of the report's conclusions."
Attorney General William Barr, who ordered the Durham investigation, said the report showed that the FBI "launched an intrusive investigation of a US presidential campaign on the thinnest of suspicions."
Mr Horowitz said his office on Monday began a new review to further scrutinise the FBI's compliance with its own fact-checking policies used to get applications to surveil US persons in counterterrorism investigations, as well as counterintelligence probes.
