In an exclusive, NBC News reported Sunday morning that Mueller has the evidence needed to charge President Trump's former national security adviser Michael Flynn. Investigators will also look at whether the elder Flynn tried to help form a plan to extradite an opponent of Turkish President Recep Erdogan who lives in the U.S.
Mr Flynn served for 24 days as Mr Trump's national security adviser but was sacked after it was discovered he had misrepresented his contacts with Sergey Kislyak, then the Russian ambassador to Washington, in a conversation with Vice President Michael Pence.
FBI investigators are reported to have arranged interviews with multiple witnesses in the coming days with the aim of gathering further evidence against Mr Flynn.
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.
On Oct. 27, White House Press Secretary Sarah Huckabee Sanders claimed the Trump administration viewed the Russian probe as "getting closer to conclusion" in an interview on Fox News' America's Newsroom.
After Trump fired James Comey in May, the former FBI director testified that the president had suggested to him that the bureau drop a potential criminal investigation into Flynn.
Flynn's lawyer, Robert Kelner, declined to comment.
Mr Flynn has undertaken paid work as a lobbyist on behalf of the Turkish government. They are former campaign manager Paul Manafort, and former campaign aides Rick Gates and George Papadopoulos.