Former White House chief of staff Reince Priebus was interviewed by Special Counsel Robert Mueller’s team Friday, according to a statement from Priebus’ attorney. “Mr. Priebus was voluntarily interviewed by Special Counsel Mueller’s team today. He was happy to answer all of their questions,” his attorney William Burck said. Special counsel investigators were expected to begin interviewing current and former White House staff regarding the Russian probe in recent weeks, two sources familiar with the matter told CNN. National Security Council chief of staff Keith Kellogg was also interviewed by Mueller’s team recently regarding former national security adviser Michael Flynn’s tenure at the White House, a source familiar with the matter told CNN. The source said they expect Mueller’s team to wrap up interviewing the first group of White House officials by the end of the month. As CNN previously reported, among the people Mueller has expressed interest in speaking with are former and current White House staffers whom investigators consider witnesses, including Priebus, former press secretary Sean Spicer, communications director Hope Hicks, White House counsel Don McGahn, communications adviser Josh Raffel and associate counsel James Burnham. Mueller’s investigators are looking for documents and emails relating to the dismissals of Flynn and FBI Director James Comey, according to sources familiar with the matter. In addition, one source confirmed that Mueller’s team wants information connected to the Oval Office meeting Trump had with Russian officials in which he bragged about firing Comey, saying it eased pressure on his White House.