(CNN) Federal investigators are examining whether former national security adviser Michael Flynn was secretly paid by the Turkish government to lobby against a critic of Turkish President Recep Tayyip Erdogan, according to a report from The New York Times.

Investigators for Special Counsel Robert Mueller asked the White House for documents related to Flynn and questioned witnesses about whether he was paid by the Turkish government, according to the Times report. The document request was not a formal subpoena, the newspaper reported.

CNN reported Thursday that Mueller's investigators have focused on Flynn's lobbying work for the Turkish government.

Flynn's lawyers declined to comment to CNN Friday night. Flynn did not comment to the Times.

Ty Cobb, the White House special counsel, told CNN: "The White House will not be discussing any specific communications with the Special Counsel out of respect for the Special Counsel and his process. Beyond that, as I have stressed repeatedly, we continue to fully cooperate with the Special Counsel."

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