Mueller’s former law firm, WilmerHale, represents Trump’s son-in-law Jared Kushner, who met with a Russian bank executive in December, and the president’s former campaign manager Paul Manafort, who is a subject of a federal investigation.

Legal experts said the ethics rule can be waived by the Justice Department, which appointed Mueller. He did not represent Kushner or Manafort directly at his former law firm.

If the department did not grant a waiver, Mueller would be barred from investigating Kushner or Manafort, and this could greatly diminish the scope of the probe, experts said.

The Justice Department is already reviewing Mueller’s background as well as any potential conflicts of interest, said department spokeswoman Sarah Isgur Flores.

Even if the Justice Department granted a waiver, the White House would consider using the ethics rule to create doubt about Mueller’s ability to do his job fairly, the sources said. Administration legal advisers have been asked to determine if there is a basis for this.