WASHINGTON/NEW YORK (Reuters) - The special counsel investigating possible collusion between President Donald Trump’s 2016 campaign and Russia has asked White House officials to preserve any records of a meeting last year between the president’s eldest son and a Russian lawyer, according to a source with knowledge of the request.

FILE PHOTO: Special Counsel Robert Mueller leaves the U.S. Capitol Building after meeting with members of the Senate Judiciary Committee in Washington, U.S., June 21, 2017. REUTERS/Aaron P. Bernstein

Special counsel Robert Mueller sent a document preservation request to the White House, saying the June 2016 meeting that Donald Trump Jr. had at Trump Tower in New York is relevant to his investigation, the source said on Friday.

The White House counsel’s office relayed the request, a routine part of the early phase of any investigation, to other members of the White House staff on Wednesday, the source said.

News earlier this month of the meeting between Trump Jr. and a Russian lawyer whom he was told had damaging information about his father’s presidential rival, Democrat Hillary Clinton, fueled questions about the campaign’s dealings with Moscow. The Republican president has defended his son’s meeting as simple politics.

Trump’s son-in-law and White House senior adviser Jared Kushner and former Trump campaign manager Paul Manafort also attended the meeting.

The Senate Judiciary Committee has called for Trump Jr. and Manafort to testify next Wednesday at a public hearing in its Russia probe.

The committee said in a statement late Friday that Trump Jr. and Manafort had agreed to negotiate with the panel over whether they will be interviewed by committee members and over documents it is seeking. The committee said it had not issued subpoenas for them to appear at Wednesday’s hearing but reserved the right to do so.

The House of Representatives Intelligence Committee said it would interview Kushner on Tuesday.

Mueller, appointed by the Justice Department in May, is probing allegations of Russian interference in the election and potential collusion by Trump’s campaign, an issue that has engulfed the six-month-old administration.

Trump has long expressed frustration with a probe that he has called a political witch hunt, and he has denied any collusion. Moscow has denied it interfered in the election campaign to try to tilt the November 2016 vote in Trump’s favor.

RUSSIAN AMBASSADOR OVERHEARD

Document requests of the type sent by Mueller generally cover emails, text messages, voicemails, notes or records. The counsel is looking for any indication that the president knew the meeting his son had was planned and might have suggested topics for discussion, the source said.

Mueller would also be inquiring into whether Trump was briefed on the meeting afterward, as well as what was discussed, the source said. Mueller would be interested, the source said, in topics such as any discussion of U.S. economic sanctions on Russia, possible Russian investments in the United States or elsewhere, or a possible lifting of a Russian ban on Americans adopting Russian children.

Russia imposed the adoption ban to retaliate for the 2012 Magnitsky Act, which imposed sanctions on Russian individuals to punish Russia for human rights violations.

Russia’s ambassador to Washington was overheard by U.S. spy agencies telling his bosses that he had discussed campaign-related matters with Trump adviser Jeff Sessions last year, the Washington Post reported on Friday, citing current and former U.S. officials.

Sessions, who was a U.S. senator at the time and is now the attorney general, initially failed to disclose the contacts with Ambassador Sergei Kislyak and then said they were not about the campaign.

Under pressure over having not disclosed the meetings with Kislyak, Sessions recused himself from the Russia probe in March. The recusal angered Trump, who said in a New York Times interview this week that he would not have chosen Sessions for attorney general if he had known Sessions would recuse himself. [nL1N1KB0UN]

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Newspaper reports said Trump’s lawyers are reviewing ways to limit or undermine the special counsel.

The Washington Post and The New York Times on Thursday cited unidentified people familiar with the strategy. A Trump attorney contacted by Reuters, John Dowd, denied the reports and praised Mueller. “We think he’s a straight, honest guy,” Dowd said.

Dowd said communications with Mueller were productive and “we have confidence he’s going to do the right thing.”

Another Trump lawyer, Jay Sekulow, told the Times that addressing possible conflicts of interest in Mueller’s team would be appropriate but declined to comment on specifics.

Senior White House adviser Kellyanne Conway referred on Friday to reports that members of Mueller’s team have made donations to the Democratic Party. “People should know what folks’ paths and motivations and political motivations are,” she said in a Fox News interview.

According to the Post, Trump has asked his advisers about his power to pardon aides, family members and even himself in connection with the Russia probe. Trump’s lawyers have been discussing the president’s pardoning powers, a second person told the newspaper.

Dowd dismissed the pardon report as nonsense. “It’s just not true,” he said.

Separately, the spokesman for Trump’s legal team, Mark Corallo, confirmed his resignation on Friday. And lead attorney Marc Kasowitz will take on a reduced role on the outside legal team, according to media reports.

Other recent changes included the hiring of veteran Washington lawyer Ty Cobb to a White House job to handle responses to the Russia probe. [nL1N1K60C4]