White House Chief of Staff John Kelly answered a narrow set of questions from the special counsel’s office related to the obstruction of justice part of the Russia investigation. Kelly joined the White House staff in the summer of 2017, making him one of the later additions to the Trump White House to be interviewed.
But the terms of the interview were also guided by Trump’s current counsel on Russia, Emmet Flood, rather than Trump’s original legal team, which had taken a more cooperative approach to responding to Mueller’s requests. CNN writes:
The Mueller questions to Kelly centered on a narrow set of issues in the investigation of potential obstruction of justice, chiefly Kelly’s recollection of an episode that took place after new reporting emerged about how the President had tried to fire Mueller. The President was angry at then-White House counsel Don McGahn about what had been reported by The New York Times. McGahn had refused to publicly deny the reporting. The special counsel wanted to try to corroborate McGahn’s version of events.
Before they allowed the interview to take place, Flood’s team stipulated that Mueller had to demonstrate that “it’s highly important” and the information can’t be obtained “anywhere else.” Otherwise, Trump’s team argued it would be improper to question a government official about official government business.
Clearly, Mueller met that standard or Kelly wouldn’t have sat for the interview.