Special counsel Robert Mueller, who is investigating Russian interference in the 2016 election and whether the Trump campaign colluded with the Kremlin, has begun questioning White House staffers as part of the probe.

Retired Lt. Gen. Keith Kellogg, the chief of staff for the National Security Council, was interviewed Thursday about former national security adviser Mike Flynn, and may have been shown records related to the investigation, according to a Fox News report Friday.

Last week, it was revealed that Mueller would be interviewing White House officials to see what they knew about Flynn's federal investigation at the beginning of 2017. He also asked for documents concerning Flynn and a Justice Department warning about Flynn's ties to a Russian diplomat.

Flynn was forced to resign from his post on Feb. 13 due to disingenuous statements about his communications with Russian officials. President Trump met with former Federal Bureau of Investigation Director James Comey in the Oval Office the following day. Comey told Congress that Trump asked him if there was a way to let Flynn go during the meeting, according to testimony from Comey over the summer.

As a result, Mueller is reportedly examining whether Trump obstructed justice in his interactions with Comey, although Trump has said he did not behave inappropriately.Flynn and Trump's former campaign manager, Paul Manafort, are two main figures of particular interest in the Mueller investigation.Mueller has reportedly told Manafort his team is working towards charging him with possible tax and financial crimes as his is investigating Manafort's actions spanning over the course of more than 10 years.