FBI director James Comey has strongly defended his decision not to recommend charges against Hillary Clinton but told Republican lawmakers penalties such as revoking security clearances could be considered for mishandling classified information as Ms Clinton and her staff did.
House of Representatives Speaker Paul Ryan said in a letter to Director of National Intelligence James Clapper that Ms Clinton, the presumptive Democratic presidential nominee, should be denied classified briefings during her campaign.
Presidential candidates normally get access to classified information once they are formally nominated and a Clinton spokesman said Republicans' efforts to deny her the briefings was "another blatantly political stunt."
Mr Comey, under persistent questioning at a hearing of the House of Representatives Oversight Committee on Thursday, said if an FBI agent had acted as Ms Clinton did, he would take their behaviour into consideration before granting their pay, employment and access to secure information.
But Mr Comey insisted that Ms Clinton had not broken the law.
"The question I always look at is, is there evidence that would establish beyond a reasonable doubt that somebody engaged in conduct that violated a criminal statute? And my judgment here is there is not," Mr Comey said.
The issue of Ms Clinton's use of private email servers has cast a cloud over her campaign leading up to the November 8 presidential election, raised questions among voters about her trustworthiness and given her Republican presidential rival, Donald Trump, an avenue of attack.
Mr Comey had disappointed some Republicans by only rebuking Ms Clinton, not recommending charges against her, in a Tuesday announcement for what he called "extremely careless" handling of classified information while using private email servers for business.
"I think there is a legitimate concern that there is a double standard, if your name isn't Clinton or you are not part of the powerful elite that Lady Justice will act differently," Representative Jason Chaffetz, chairman of the House Oversight Committee, said to Mr Comey.
Mr Comey said his FBI team had conducted its investigation "in an apolitical and professional way" and he had no reason to believe Clinton had lied to the FBI. Clinton had said publicly she never sent or received any classified information.
Mr Comey's testimony to the House Oversight Committee marked the first time he has taken questions publicly since his announcement this week that the FBI was not recommending charges against Clinton.
US Attorney-General Loretta Lynch has accepted the FBI recommendation and was to appear at another hearing next Tuesday.
Mr Comey said on Tuesday that 110 classified emails passed through Ms Clinton's servers, which were not kept on a secure government server.
He rebuked her for "extremely careless" handling of classified information but said no reasonable prosecutor would charge her or her staff criminally.
Mr Comey, a Republican who was appointed by Democratic President Barack Obama and also served in the administration of former Republican President George W Bush, has built a reputation as a straight shooter who does not bend to pressure from either party.