Special counsel Robert Mueller is in the process of arranging interviews with current and former Trump administration officials as part of his ongoing investigation into Russia’s interference in the 2016 election, the New York Times reported over the weekend.

Two people briefed on the negotiations told the Times that Mueller is particularly interested in learning more about key meetings, such as any discussions that led up to the abrupt May dismissal of former FBI director James Comey. Mueller reportedly assumed control of an FBI investigation into whether President Donald Trump obstructed justice by firing Comey, who oversaw the Russia investigation at the time. Trump later bragged to top Russian diplomats that doing so took “great pressure” off of him.

Ty Cobb, one of the lawyers leading Trump’s outside counsel, told the Times that the White House would “continue to fully cooperate” with the investigation.

Recently ousted chief of staff Reince Priebus is at the top of the list of Trump associates Mueller wants to speak with, according to the Times’ report. As chairman of the Republican National Committee, Priebus began working closely with Trump’s campaign as soon as he emerged as the presumptive GOP nominee, and he was a fixture at most major meetings through the transition and during the first six months of the administration.

The Times report revealed that Priebus was also on the calendar of former Trump campaign chairman Paul Manafort on June 9, 2016—the day that Manafort and other officials met with Russians who claimed to have damaging information about Hillary Clinton to offer as part of the Kremlin’s efforts to help the Trump campaign. Whether the two actually met, and what they discussed if they did meet, will be important for Mueller to uncover.

That Mueller wants to interview former and current officials promises another headache for a White House inundated with them. Trump was heavily criticized last week for his seemingly off-the-cuff escalation of threats against North Korea, in which he promised the U.S. could take military action against Kim Jong-un’s government at any time. The President has also taken repeated aim at his own Senate majority leader, Mitch McConnell (R-KY), whose cooperation he will desperately need as the fall legislative session begins and he hopes to finally make progress on his stalled agenda.