US Attorney General Jeff Sessions has been questioned in an investigation into whether Russia colluded with the Trump campaign in the 2016 election.
The Department of Justice (DoJ) confirmed to the BBC that America's top prosecutor was interviewed last week.
Mr Sessions is thought to be the first member of President Donald Trump's cabinet to be questioned.
Former FBI Director Robert Mueller is leading the inquiry into alleged Russian meddling in the 2016 election.
Mr Mueller, who was appointed as special counsel, is also investigating whether Mr Trump's firing of FBI director James Comey last year was an effort to obstruct the agency's Russia probe.
The interview with Mr Sessions, the nation's top lawman, lasted several hours, according to US media reports.
The US intelligence community has already concluded that Moscow tried to sway the presidential election in favour of Mr Trump, though Russia denies this.
Special Counsel Mueller, as well as three different congressional committees, is investigating the allegation of Russian meddling, and possible collusion.
Heading toward a face-off with Trump?
Anthony Zurcher, BBC News, Washington
Robert Mueller's investigation has now reached into Donald Trump's political cabinet.
While this is a significant development, it shouldn't come as a major surprise given that Jeff Sessions sits at the intersection of multiple reported threads of the inquiry. The former senator had contact with Russians while he was senior adviser to the Trump campaign. Later he was less than forthcoming about their nature.
Mr Sessions also may know details about why the president fired FBI Director James Comey, which could be a key component of any obstruction of justice case Mr Mueller builds against the president or his staff.
With the Sessions interview - and recent news that Steve Bannon will also speak with Mr Mueller - there are only a handful of top Trump advisers who served in both the administration and the campaign who have yet to receive a visit from the special counsel's office. Vice-President Mike Pence is one. Donald Trump Jr and Ivanka Trump are two others.
Then there's the president himself.
Mr Mueller's investigation appears to be heading toward a face-off with the president. Then it will be time for the former top FBI man to put his cards on the table.
Why was Sessions questioned?
Mr Sessions, who oversees the FBI at the DoJ, recused himself from the investigation in March 2017 after he acknowledged two previously undisclosed meetings with a Russian ambassador during the campaign.
The president repeatedly criticised Mr Sessions for his recusal on Twitter.
Sessions and TrumpImage copyrightGETTY IMAGES
The former Alabama senator could be an important witness due to his ties to the firing of Mr Comey.
The White House initially said that the FBI director's sacking was on the recommendation of the DoJ, citing a departmental memo faulting Mr Comey for his handling of the investigation into Hillary Clinton's use of a private email server.
Is Trump closer to obstruction of justice?
But Mr Trump later said he was thinking of "this Russia thing" when he fired Mr Comey.
Who has been interviewed?
It is not publicly known exactly who has and has not been interviewed as part of Mr Mueller's probe. However, several names have been reported in US media.
George Nader, a little-known Steve Bannon associate who boasts of his well-placed connections in the Middle East, was interviewed at least twice, according to Axios.
Mr Trump's lawyers were reportedly in talks with investigators in December about a possible interview with the president.
Mueller 'in talks to interview Trump'
Who has been charged?
Four people have been criminally charged as part of Mr Mueller's investigation.
Michael Flynn, the president's former national security adviser, pleaded guilty to lying to the FBI about a meeting with a Russian ambassador.
Former Trump campaign manager Paul Manafort has been charged on 12 counts, including conspiring to defraud the US in his dealings with Ukraine, and conspiracy to launder money.
Mr Manafort's business associate Rick Gates was also charged with conspiracy to launder money.
A third adviser to the campaign - George Papadopoulos - pleaded guilty to lying to the FBI.
Is the FBI impartial?
The White House has repeatedly argued that FBI agents are biased against Mr Trump, who claimed last year the bureau was in turmoil.
Republicans in Congress are claiming to have a secret memo which alleges that agents illegally spied on Mr Trump during Barack Obama's presidency.
So far they have yet to release the four-page secret memo, which Republicans say includes allegations that may warrant criminal charges.
Why attacks on Mueller are mounting
The news of Mr Sessions interview comes a day after Axios reported that FBI Director Christopher Wray threatened to resign as he was allegedly pressured by Mr Sessions to fire his deputy, Andrew McCabe.
Mr McCabe served as acting FBI director before the president tapped Mr Wray to replace Mr Comey.
Mr Trump has criticised Mr McCabe for the FBI investigation into Mrs Clinton's use of a private email server, pointing out that his wife ran for Virginia state office as a Democrat during the inquiry.
Andrew McCabeImage copyrightGETTY IMAGES
Image caption
FBI Director Wray reportedly threatened to quit over pressure to fire his deputy, Andrew McCabe (pictured)
Former Governor Terry McAuliffe, a Clinton ally, donated to the failed campaign of Mr McCabe's wife.
But the FBI concluded earlier this month that Mr McCabe had no conflicts in the Clinton investigation.
In response to the Axios report, White House spokesman Raj Shah said that Mr Trump "believes politically-motivated senior leaders including former Director Comey and others he empowered have tainted the agency's reputation for unbiased pursuit of justice".
"The President appointed Chris Wray because he is a man of true character and integrity, and the right choice to clean up the misconduct at the highest levels of the FBI," he added.