FBI director James Comey has confirmed the security agency is investigating allegations that Russia had an influence over President Trump's campaign ahead of last year's election, and revealed the probe began last July.
Comey was testifying to the US House of Representatives intelligence committee, along with National Security Agency director Admiral Mike Rogers.
It is unusual for the FBI to disclose any information about ongoing investigations, but Comey said he had been authorised by the US Justice Department to do so.
"Because it is an open, ongoing investigation and is classified, I cannot say more about what we are doing and whose conduct we are examining," Comey said.
Both Comey and Rogers said there was no evidence that Russian hackers had tampered with votes in the November election.
However, Rogers told the committee he stands by intelligence reports from earlier this year that Russia interfered with last year's election to see Donald Trump elected as President.
Meanwhile, Comey said neither the FBI nor the Department of Justice has any evidence to support Trump's wiretapping claims.
Earlier this month the President accused his predecessor Barack Obama of tapping his phones in Trump Tower. He made the allegations in a series of tweets.
"With respect to the President's tweets about alleged wiretapping directed at him by the prior administration, I have no information that supports those tweets," Comey said today.
Meanwhile, White House press secretary Sean Spicer accused the UK's GCHQ of spying on Trump, a charge the British government strongly denied last week. On Friday, the US was reported to have issued a formal apology to the UK about the allegations, although Spicer later denied these reports.
Today, Rogers said there was no evidence to back up the accusations against the UK, and described the allegations as "nonsense".