The FBI is now probing possible cooperation between the Trump campaign and Moscow's meddling in the 2016 presidential election, according to FBI Director James Comey.
British spy agencies, along with other European intelligence counterparts, were the first to intercept what could be identified as suspicious communications between associates of President Donald Trump and Russian officials, CNN reported Thursday.
"If foreign intelligence agencies share information with United States intelligence, and it's relevant to the investigation, then of course the intelligence committee will look at it", a source close to the Senate investigation told CNN.
The details were said to have been passed to the Americans as part of the routine exchange of information between the two intelligence communities.
Toward the end of 2015, the UK Government Communications Headquarters (GCHQ) received information on "suspicious interactions" between figures close to Trump and people suspected of being Russian spies, the newspaper reported, citing sources close to UK intelligence.
British and European intelligence agencies, including GCHQ, the British intelligence agency responsible for communications surveillance, were not proactively targeting members of the Trump team but rather picked up these communications during what's known as "incidental collection", these sources tell CNN.
Brennan reportedly established a counterintelligence task force, which included the Federal Bureau of Investigation and the National Security Agency.
They learned this because they were spying on those Russian agents anyway. Because the CIA deals exclusively with foreign intelligence, it did not have the authority to announce the evidence it apparently had of such collusion. Trump was not named in the warrant, but three of his associates were the subject of the inquiry.
The claims are highly sensitive after Mr Trump last month alleged that President Barack Obama had illegally "wiretapped" him in Trump Tower.
The comment prompted a near-immediate reaction from GCHQ, which released a rare public statement categorically denying any involvement and calling the accusation "utterly ridiculous".