Mr Mueller's team are also said to be looking at whether the former National Security Adviser attempted to have Fethullah Gullen, an elderly Muslim cleric, extradited from the U.S. to Turkey in exchange for millions of dollars.
Lt. Gen. Michael T. Flynn served as Trump's national security adviser for just 24 days until he was forced to quit due to accusations he had contact with Russia's ambassador to the United States and misled the White House about the conversations.
If charges are brought against Flynn, he would be the first former or current member of the Trump administration to be formally indicted in the Russian probe.
The investigators will be questioning multiple witnesses in coming days to gain more information on Flynn's lobbying activities. Shortly afterward, Flynn's son tweeted a 2013 photo of his father and Mueller sitting at the same table during a Senate Intelligence Committee hearing: "A picture worth a thousand words.go nuts with your comments".
Three House Republicans have said "obvious conflicts of interest" should require Mueller to step down, citing his relationship with former Federal Bureau of Investigation director James Comey, Politico reports.
Over the course of Mueller's investigation, Ryan has said people should let Mueller do his job and defended Mueller from critics. But cooperation from Flynn, who was national security adviser for only 24 days before he was sacked, could help his son avoid the worst penalties. It was also revealed last week that former foreign policy adviser for the Trump campaign, George Papadopoulos, had pleaded guilty to lying to the F.B.I.
"If the elder Flynn is willing to cooperate with investigators in order to help his son, two of the sources said, it could also change his own fate, potentially limiting any legal consequences", NBC reported.