Special counsel Robert Mueller has interviewed former acting Attorney General Sally Yates about meeting with White House officials where she expressed concern over Donald Trump’s onetime national security adviser Mike Flynn, the Wall Street Journal reports.

The interview reportedly took place in July, and indicates the special counsel is probing how much the White House knew about the FBI’s investigation into Flynn. Mueller has also reportedly expressed interest in Trump’s decision to fire former FBI Director James Comey, signaling a potential obstruction of justice investigation.

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In May, Yates testified that she warned the president just a week into his presidency that Flynn was vulnerable to blackmail by the Russians. The White House waited 18 days to fire Flynn, doing so only after it was revealed he lied to Mike Pence about the nature of his conversations with Russian ambassador Sergey Kislyak.

On Dec. 1, Flynn pleaded guilty to lying to the FBI about that contact.