The Clinton campaign has blasted FBI Director James Comey for "blatant double standards" over the new inquiry into Hillary Clinton's email use.
The comments came after US media reports that Mr Comey had urged against publicly accusing Russia of interfering in the US election, including alleged email hacking.
Mr Comey's concern about releasing the information was due to the proximity to the election, reports say.
The statement that Mr Comey reportedly declined to sign off on was released by the Department of Homeland Security and the Office of the Director of National Intelligence on 7 October.
"The US intelligence community is confident that the Russian Government directed the recent compromises of emails from US persons and institutions, including from US political organisations... these thefts and disclosures are intended to interfere with the US election process," it said.
Mr Comey agreed with the statement but was against making it public before the election, according to US media.
There have been allegations that Russian hackers have targeted the Democrats in an effort to skew the election in favour of Donald Trump.
"It is impossible to view this as anything less than a blatant double standard," Clinton campaign manager Robby Mook said. He also called on Mr Comey to "immediately explain this incongruence and apply the same standard to Donald Trump's associates as he has applied to Hillary Clinton's."
Mr Comey has faced a fierce backlash for announcing on Friday, just 11 days before the presidential election, that the FBI is investigating new emails that may be linked to its probe into Mrs Clinton's private email server.