The special counsel investigating possible collusion between 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 son and a Russian lawyer, according to a source with knowledge of the request.

The special counsel, Robert Mueller, sent a document preservation request to the White House, saying the June 2016 meeting that Donald Trump Jr had in 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 staff on Wednesday, the source said.

Q&A What is the Magnitsky Act? Show Sergei Magnitsky was a Russian lawyer who investigated a massive $230m tax fraud in 2007.

After he revealed the scam, Magnitsky was arrested by the same officials whom he had accused of covering up the racket and was imprisoned. He died in jail after being denied medical treatment. Russia accused him of committing the fraud himself and even put him on trial posthumously.

After a long campaign by his associates, the Magnitsky Act was passed by Congress in 2012, banning entry to the US and freezing assets of officials believed to have been involved in Magnitsky's persecution and death. Russia responded by banning a list of US citizens it deemed hostile to Russia, and blocking the adoption of Russian children by US citizens – a controversial move that led critics to suggest the Kremlin was punishing Russia’s most vulnerable children.

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

Appointed by the justice department in May, Mueller is investigating US intelligence agency allegations that Russia 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 an investigation that he has called a political witch-hunt, and 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.

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.

He 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 US 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.

Late Thursday, a Washington Post article reported that Trump was consulting with advisers “about his power to pardon aides, family members and even himself” in connection to the Russia probe.

And both the New York Times and the Associated Press reported that Trump’s legal team were looking for conflicts of interest among members of Mueller’s investigative team, as part of an effort to discredit the investigation, or even get Mueller fired.

Also on Friday, the co-founder of the firm that commissioned a dossier on Donald Trump during the presidential campaign last year is on vacation and will not testify at a Senate judiciary committee hearing on the Russia probe next week, his lawyers said in a letter released on Friday.

Glenn Simpson, of Fusion GPS, is on vacation through 31 July and will be traveling abroad through 3 August, his attorneys said in the letter, adding that they were “profoundly disturbed” that the hearing had been expanded due to “partisan agendas” to include allegations of Russian meddling in the US election and possible ties between the Trump campaign and Russia.

Donald Trump Jr and President Trump’s one-time campaign manager Paul Manafort were due to testify on the same day as Simpson. Simpson’s attorneys asked that he be excused from appearing, adding that allegations he had failed to register as a foreign agent were “nothing more than an effort to smear him”.

