(CNN) Special counsel investigators could start interviewing current and former White House staff as soon as later this week regarding the Russian probe, two sources familiar with the matter tell CNN.

One source cautioned it is still being worked out with Robert Mueller's office and said it might be delayed until next week.

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 former chief of staff Reince 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 team also approached the White House about interviewing staffers who were aboard Air Force One for the creation of the initial response to news of Donald Trump Jr.'s Trump Tower meeting with a Russian lawyer, sources previously told CNN

Mueller's investigators are looking for documents and emails relating to the dismissals of national security adviser Michael Flynn and FBI Director James Comey, according to the sources. In addition, one source confirms 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.

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