Special counsel Robert Mueller has reportedly started interviewing White House officials as part of the investigation into ties between Trump campaign associates and Russia.

Fox News reported Friday that retired Lt. Gen. Keith Kellogg, the chief of staff for the National Security Council, was interviewed on Thursday. According to the report, Kellogg was shown information and asked questions related to President Trump's former national security adviser Michael Flynn.

Flynn resigned in February after reports he misled senior members of the White House about his contacts with Russian officials.

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Fox also reported Friday that congressional investigators are looking at a Trump campaign meeting in March 2016 about national security that took place at the site of what is now Trump's hotel in Washington, D.C.

During the meeting, a staffer named George Papadopoulos reportedly suggested setting up a meeting with senior Russian officials about repairing the U.S.-Russia relationship.

That idea, according to Fox, was immediately shot down by then-Sen. Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE (R-Ala.). Others raised concerns it would violate the Logan Act, which prohibits Americans from engaging with a foreign government without authorization from U.S. officials.

Prosecutors from Mueller's team reportedly told former Trump campaign manager Paul Manafort to expect an indictment when they raided his home with FBI agents in July, a sign that the investigation is speeding up.

Manafort, who was fired from the campaign last summer, is another central figure in the special counsel probe as prosecutors continue to scrutinize his possible ties to Russia, lobbying work overseas and other previous business transactions dating back as far as 11 years.