US President Donald Trump lashed out at the FBI, issuing a fresh denial that he asked former director James Comey to drop an investigation into the conduct of one of his top aides, Michael Flynn.
In a Twitter tirade on Sunday, Trump said the FBI's reputation was "in tatters".
His attack came amid a flurry of developments in Special Counsel Robert Mueller's inquiry into alleged Russian interference in the US election.
Trump denies that his team colluded with Russia to get him elected, according to BBC.
Reports emerged over the weekend that Mueller, a former FBI director, had dismissed an FBI officer from the investigation during the summer after he was discovered to have made anti-Trump remarks in text messages.
The president seized on the officer's dismissal, tweeting: "Report: 'ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE' Now it all starts to make sense!"
A spokesman for Mueller said the officer was dismissed from the investigating team as soon as the messages were discovered.
Michael Flynn, the president's former national security adviser, announced on Friday that he was co-operating with Mueller's investigation, in return for pleading guilty to a lesser charge.
The former general admitted lying to the FBI and has been offered a reduced sentence of six months. Analysts say the deal indicates that Flynn has incriminating information about one or more senior members of the Trump administration.
In a series of tweets posted on Sunday morning, Trump again attacked his former rival for the presidency, Hillary Clinton, who was investigated by the FBI ahead of the election after it emerged she had used a private email server to conduct state department business.
No charges were brought against Mrs Clinton or her team.
In another tweet, the president accused ABC News of "horrendously inaccurate and dishonest reporting", after one of the network's reporters acknowledged making an error in a story about the president.