(CNN) 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.

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