WASHINGTON (Reuters) - Acting U.S. Attorney General Matthew Whitaker has decided not to recuse himself from overseeing the investigation into Russian meddling in the 2016 presidential election, disregarding advice from his own ethics officials, a high-ranking Justice Department official said on Thursday.

The decision by Whitaker, known for making comments critical of Special Counsel Robert Mueller’s Russia probe before being appointed last month by President Donald Trump, was conveyed in a letter to congressional leaders by Assistant Attorney General Stephen Boyd.

Boyd said in the three-page letter department ethics officials determined that Whitaker lacked any personal, political or business conflicts that would disqualify him from supervision of Mueller’s investigation.

Ethics officials concluded, however, that if their recommendation were sought “they would advise that the acting attorney general should recuse himself” because “a reasonable person with knowledge of the relevant facts likely would question” Whitaker’s impartiality, Boyd wrote.

Boyd said the “appearance-of-impartiality” test in Whitaker’s case was deemed a “close call” in which “credible arguments could be made either way,” a finding he said Whitaker cited in his decision to retain his discretion to oversee the Mueller investigation.

“The ultimate decision about whether or not to recuse from a matter in a case such as this rests with the acting attorney general,” Boyd said.

The letter, obtained by Reuters, was addressed to the top Republican in the U.S. House of Representatives, Speaker Paul Ryan, and House Democratic leader Nancy Pelosi.

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Mueller’s investigation, which Trump has derided as a “witch hunt,” is also examining whether Trump’s election campaign had colluded with Moscow and any possible obstruction of justice.

The probe has already ensnared Trump’s former campaign manager, former personal lawyer and his former national security adviser. Trump has denied wrongdoing and Moscow has said there was no interference.

The president’s frequent criticism of the probe has raised concerns he may attempt to shut it down, putting a spotlight on the top Justice Department officials overseeing it.

It is unclear how long Whitaker, a former U.S. attorney and conservative commentator, will head the department.

His appointment, immediately following Trump’s ouster of Jeff Sessions as attorney general in November, is under challenge in several court cases contending that the president violated the constitution by installing Whitaker without Senate confirmation.

Trump has already picked former Attorney General William Barr to become the department’s new permanent chief, but the nomination needs to be approved by the Senate.

The top Democrat in the Senate, Chuck Schumer, said on Thursday Barr was unfit to serve given a memo he wrote to the Justice Department arguing that Mueller should not be permitted to look into possible attempts by Trump to obstruct the investigation.

“The president must immediately reconsider and find another nominee who is free of conflicts and will carry out the duties of the office impartially,” Schumer said.