- Video report by ITV News Washington Correspondent Robert Moore
US President Donald Trump has said he is the victim of the "single greatest witch hunt of a politician in American history".
Mr Trump tweeted the claim as he reacted angrily to the appointment of former FBI Director Robert Mueller as special counsel to lead an investigation into alleged collaboration between Mr Trump's election campaign team and Russia.
Mr Trump later condemned the appointment, which was made after he sacked former FBI director James Comey last week, claiming Hillary Clinton and Barack Obama had not faced the same scrutiny for "illegal acts".
The president spoke out as it emerged his former national security adviser is refusing a legal demand to hand over private documents to a separate probe investigating links between the Kremlin and Mr Trump's associates.
The head of the Senate Intelligence Committee said Michael Flynn's lawyer confirmed he will not honour the committee's subpoena for the files.
Congressional aides told Reuters the committee is still negotiating with Mr Flynn to obtain the documents.
The FBI, Senate and House intelligence committees and are all conducting their own investigations into claims of collusion between Russia and Mr Trump's 2016 campaign.
On yet another hectic day of developments in Washington, the chairman of the House committee examining Mr Trump's decision to remove Mr Comey from the FBI is expected to announce he will leave Congress next month.
The House Oversight Committee that Republican Representative Jason Chaffetz leads is also investigating whether Mr Flynn misled security clearance investigators about his contacts with Russia.
His impending departure was reported by Politico, citing unnamed sources.