Donald Trump's former campaign manager Paul Manafort (pictured) was forced to resign from the campaign in June 2016. Drew Angerer/Getty Images

WASHINGTON (Reuters) — U.S. investigators examining money laundering accusations against President Donald Trump’s former campaign manager Paul Manafort hope to push him to cooperate with their probe into possible collusion between Trump's campaign and Russia, two sources with direct knowledge of the investigation said.

Special Counsel Robert Mueller's team is examining Manafort's financial and real estate records in New York as well as his involvement in Ukrainian politics, the officials said.

Between 2006 and 2013, Manafort bought three New York properties, including one in Trump Tower in Manhattan. He paid for them in full and later took out mortgages against them. A former senior U.S. law enforcement official said that tactic is often used as a means to hide the origin of funds gained illegally. Reuters has no independent evidence that Manafort did this.

The sources also did not say whether Mueller has uncovered any evidence to charge Manafort with money laundering, but they said doing so is seen by investigators as critical in getting his full cooperation in their investigation.

"If Mueller's team can threaten criminal charges against Manafort, they could use that as leverage to convince him to cooperate," said one of the sources.

Manafort’s spokesman, Jason Maloni, said, “Paul Manafort is not a cooperating witness. Once again there is no truth to the disinformation put forth by anonymous sources and leakers.”

A Brooklyn brownstone, allegedly owned by Paul Manafort. Spencer Platt/Getty Images Manafort is seen as a key figure in the investigation because of his senior role in the campaign and his participation in a June 2016 meeting that included the president's son, Donald Trump Jr., close adviser Jared Kushner and Russian lawyer Natalia Veselnitskaya.

The meeting was called after the lawyer offered damaging information about Democratic presidential candidate Hillary Clinton.

Mueller's team asked the White House on Friday to preserve all of its communications about that meeting. Mueller is examining contacts between Russian officials and Trump associates during and after the Nov. 8 presidential election as part of a broader investigation into whether Russia tried to sway the election in favor of Trump.

Manafort became Trump's campaign manager in June 2016 but was forced to resign two months later amid reports of his business relationship with the Kremlin-backed former Ukrainian leader, Viktor Yanukovich.

Manafort previously worked as a consultant to a pro-Russia political party in Ukraine and helped support Yanukovich. According to a financial audit reported by the New York Times, he also once owed $17 million to Russian shell companies.

Former Southern District of New York U.S. Attorney Preet Bharara was investigating Manafort's real estate dealings before he was fired by Trump in March, and Mueller has now assumed control of that investigation, one of the sources said.

Bharara was not available for comment on his investigation on Friday.