The Justice Department's inquiry is separate from the criminal investigation into Giuliani's activities in Ukraine, the source said.
Giuliani's actions first raised questions in 2017 when he was hired to represent Turkey-based, Iranian-born businessman Reza Zarrab in a money laundering and sanctions evasion case brought against Zarrab by the U.S. attorney in the Southern District of New York.
Sources also said at the time that Giuliani urged the administration to deport exiled cleric Fethullah Gulen, whom Turkish President Erdogan accused of inciting a coup.
An attorney for Giuliani could not be reached for comment, but the former New York City mayor has previously denied any wrongdoing.
A spokesperson for the Justice Department declined to comment, as did a spokesperson for the U.S. Attorney’s Office for the Southern District of New York.
It wasn’t immediately clear when this inquiry, first reported by Bloomberg, began.