Unnamed law enforcement and intelligence sources confirmed to NBC News on Monday that the FBI is conducting a preliminary examination of Manafort's foreign business connections.
Manafort told NBC News that he was not aware of any investigation by the FBI, and said that any suggestion that he is connected to Russian President Vladimir Putin or his government is "Democratic propaganda” and "meant to deflect."
The FBI did not comment on NBC News' report.
In August, the New York Times reported that a ledger kept by the political party of deposed Ukrainian President Viktor F. Yanukovych showed $12.7 million in cash payments to Manafort.
Manafort denied accepting the payments, calling the report "unfounded, silly, and nonsensical."
A report published by the Associated Press that same week revealed that Manafort's firm ran a lobbying operation in the United States on behalf of Ukraine's ruling political party to obtain positive media coverage of the country's pro-Russian government. Manafort had not disclosed his work as a foreign agent as required under federal law. He did not comment on that Associated Press story.
Manafort resigned from the Trump campaign days later. Kellyanne Conway, Manafort's replacement, has said that he "was asked" to resign.
Ousted Trump campaign manager Corey Lewandowski, Manafort's predecessor, had suggested that those reports were a factor in Manafort's resignation, citing the "potential distraction of more stories coming" about his overseas ties.