The FBI director gave inaccurate information about Hillary Clinton’s emails to Congress last week, say sources at the agency.
James Comey told lawmakers that Mrs Clinton’s top aide, Huma Abedin, forwarded “hundreds and thousands” of classified emails to her then-husband.
However, investigating officials say the emails were only a handful in number and not classified.
Mrs Clinton has blamed Mr Comey for her election defeat to Donald Trump.
He made two interventions during the election campaign, first in July and again in October, to make pronouncements about the FBI investigation into Mrs Clinton.
Appearing in front of the Senate Judiciary Committee, he said: “Huma Abedin appears to have had a regular practice of forwarding emails to him [then-husband Anthony Weiner] for him I think to print out for her so she could then deliver them to the secretary of state.”
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Mrs Clinton has blamed Mr Comey by name for her election loss
At another point, he said the adviser “forwarded hundreds and thousands of emails, some of which contain classified information”.
But sources close to the FBI investigation have told US media Ms Abedin only sent a handful of emails to her husband for printing, and they were not marked classified at the time.
Officials tell ProPublica that some of Ms Abedin’s emails may have wound up on her husband’s computer as a result of a backup of her work Blackberry device.
The FBI is trying to determine how to notify Congress of the director’s misstatement.
Mr Comey told the panel of lawmakers that it makes him “mildly nauseous” to think that he could have had an impact on the US presidential election.
In July he said there were no charges for Mrs Clinton to face, and then told Congress four months later he was reopening the investigation in order to study extra emails found on Mr Weiner’s computer.
She was cleared, for a second time, days before the election.
FBI director ‘misstated’ key Clinton email claim, sources say