WASHINGTON (Reuters) - U.S. Attorney General Jeff Sessions was questioned last week by the special counsel’s office investigating potential collusion between Russia and President Donald Trump’s 2016 presidential campaign, the U.S. Justice Department said on Tuesday.

The interview marked the first time that Special Counsel Robert Mueller’s office is known to have interviewed a member of Trump’s Cabinet, and is another milestone in an investigation that has hung over Trump’s year-old presidency.

Discussions between Trump’s personal lawyers and Mueller’s team have been under way about the possibility of an interview with Trump and what the scope of it might be, according to a person familiar with the talks.

Mueller’s office also interviewed former Federal Bureau of Investigation Director James Comey shortly after Trump fired Comey in May 2017, a person familiar with the matter said. Comey’s firing led to Mueller’s appointment to take over the FBI’s Russia investigation.

Sessions was the first U.S. senator to endorse Trump’s candidacy and served as a campaign adviser before the Republican president appointed him as the top U.S. law enforcement official.

Trump has openly criticized Sessions for recusing himself from overseeing the Russia probe last March after media reports that he had failed to disclose 2016 meetings with Moscow’s then-ambassador, Sergei Kislyak.

Ian Prior, a Justice Department spokesman, confirmed a report in the New York Times that Sessions met for hours with Mueller’s team last week. Prior did not provide additional details. An attorney representing Sessions declined to comment.

FILE PHOTO: U.S. Attorney General Jeff Sessions speaks during a news conference to discuss "efforts to reduce violent crime" at the Department of Justice in Washington, U.S., December 15, 2017. REUTERS/Joshua Roberts/File Photo

Mueller’s team is expected to be interested in meetings between Sessions and Kislyak during the campaign, as well as the attorney general’s involvement in Trump’s firing of Comey, an episode central to the question of whether Trump may have committed obstruction of justice.

Trump shrugged off questions about Sessions’ interview at the White House, telling reporters: “I’m not at all concerned.” His spokeswoman Sarah Sanders declined comment on specific interviews but said the White House would not stand in the way of the probe.

“We’re going to be fully cooperative with the special counsel,” Sanders told reporters. “We want to see this come to a complete and full conclusion.”

QUESTIONS FOR TRUMP?

U.S. intelligence agencies have concluded that Russia interfered in the 2016 campaign using hacking and propaganda to attempt to tilt the race in favor of Trump. Russia has denied it. Trump has denied any collusion with Russia, and has called the Mueller’s investigation a “witch hunt” and “hoax.”

Mueller wants to question Trump himself in coming weeks about decisions to fire national security adviser Michael Flynn - who had lied about contact with Kislyak - and Comey, the Washington Post said in a report.

Trump refused this month to commit to being interviewed by Mueller, saying: “I’ll speak to attorneys” about the matter.

Trump’s personal lawyer John Dowd declined comment on Tuesday.

A person familiar with Comey’s interview with Mueller’s team said it was part of the handoff of the investigation into whether Trump sought to obstruct justice by firing Comey, and whether the president’s campaign colluded with Russia.

The handoff, the source said, also included Mueller’s team collecting all the material Comey had gathered during the initial stages of the investigation.

In memos Comey wrote about his meetings with Trump, Comey recounted how the president asked him to end an FBI probe into Flynn, who has since pleaded guilty to a charge brought by Mueller of lying to the FBI.