Donald Trump is under fire for saying the FBI’s reputation is “in tatters” over its probe into alleged collusion between Russia and his campaign.
Ex-FBI head James Comey and ex-attorney general Sally Yates, who were both fired by Mr Trump, led the criticism.
Republican senator Lindsey Graham said his comments and tweets on ongoing investigations were at his “own peril”.
Meanwhile there have been suggestions the US president may have admitted obstructing justice in his tweets.
Mr Trump posted a barrage of criticism on Sunday morning, saying the FBI’s reputation was “in tatters” and was the “worst in history”, while again accusing it of failure in its treatment of his former opponent for the presidency, Hillary Clinton.
Mrs Clinton was investigated by the FBI ahead of the election after it emerged she had used a private email server to conduct state department business, but no charges were brought against her or her team.
The president seized on news that an FBI officer had been dismissed from the investigation after he was discovered to have made anti-Trump remarks in text messages, tweeting: “Report: ‘ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE’ Now it all starts to make sense!”
Mr Trump denies that his team colluded with Russia to get him elected, but four members of his inner circle have now been charged as part of the FBI inquiry lead by Robert Mueller.
Former acting attorney general Sally Yates hit back at Mr Trump, tweeting that “the dedicated men and women of the FBI deserve better” and that the “only thing in tatters is the president’s respect for the rule of law”.
The former director of the FBI, James Comey, who was fired by President Trump and has testified in the investigation, posted a strong rebuttal of the president’s criticism of the organisation.
He posted online a quote from himself in June that said: “I want the American people to know this truth: The FBI is honest. The FBI is strong. And the FBI is, and always will be, independent.”