For close to 20 years, Felix Sater lived a double life.

A brash real estate developer and longtime associate of the Trump organisation, he was doing deals in Donald Trump's name during the 2016 presidential campaign.

Sater was also secretly working as an informant and asset for multiple US intelligence and law enforcement agencies, helping track down and catch mafia gangsters, cyberhackers and terrorists.

In his own words, Sater was "building Trump Towers by day and hunting Bin Laden by night".

Four Corners travelled to New York to speak with Sater about his business dealings with Mr Trump, which are now part of special prosecutor Robert Mueller's investigation into Russian interference in the 2016 election.

Sater claims he is just an ordinary man who has lived an extraordinary life.

"In 20 years I've done a variety of many different things with the US government, both with Department of Justice, CIA, FBI, Secret Service," he said.

"Sometimes very common people wind up in very uncommon situations."

Among the cast of unusual characters in Mr Trump's orbit, Sater's story stands out. Recently unsealed court documents co-signed by former President Barack Obama's attorney-general Loretta Lynch validate his incredible tale.

"[Felix Sater] provided … information crucial to national security and the conviction of over 20 individuals — including those responsible for committing massive financial fraud and members of La Cosa Nostra … he passed on specific information about key leaders in Al Qaeda and affiliated groups, including information that could help the United States locate those individuals," the documents read.

Sater's family emigrated to the US from Russia when he was seven years old. He grew up near Brighton Beach in Brooklyn, New York City, an area notorious for its concentration of Mafia gangsters and Russian mobsters. At 18, Sater worked for Wall Street trading firm Bear Stearns.

In 1991 in a bar fight, Sater attacked another man with a broken margarita glass. The victim needed more than 100 stitches and 25-year-old Sater was convicted of assault.

After 15 months behind bars and no longer legally able to operate as a broker, Sater participated in a multi-million-dollar stock fraud.

'Everybody's got a guy'

He denies being a mobster but says there was a time in his life when he became entangled with organised crime.

"As we used to say growing up, 'everybody's got a guy'," he told Four Corners.

"There was an incident in my life they were involved in but that doesn't make me mob linked."

As part of a deal with the Justice Department, he avoided jail time for the stock fraud by becoming an asset for US law enforcement. Sater claims his work for the US government had already begun.

"Everyone says that I did it to get out of jail. That's not true, I did it before anything ever happened," he said.

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Trump biographer Tim O'Brien questions the deal struck by Sater.

"He would have gone to jail but for the fact that Felix had a lot of interesting information that the US government wanted," he said.

"The interesting thing about that is how did Felix get that information and where did he get it? Felix had gone back to Russia, had gone to Moscow and obviously had contacts somewhere in the Russian government, where he was able to get not only credible, but actionable information."

O'Brien and Sater have history. In 2009 Sater testified as a witness for Mr Trump in his long-running $5 billion libel lawsuit against O'Brien.

Mr Trump had alleged O'Brien had deliberately underestimated the businessman's net worth in his 2005 book, Trump Nation.

Mr Trump eventually lost that lawsuit in 2011.

Sater assisted authorities for another two decades. The 52-year-old told Four Corners one of his first missions was tracking down missing Stinger missiles. In the late 1990s he went as far as helping the US government track Osama bin Laden.

"Not only did I contribute to finding his location, I delivered his satellite phone numbers which we used to then listen in on him. Subsequently President Bill Clinton used the coordinates off those satellite phone numbers to home in and authorise the bombing of his camps in I believe it was '98," he said.

Felix Sater with Donald and Melania Trump. ( Supplied: Felix Sater )

The Trump deals

In 2000, while his work for US authorities continued, Sater joined a real estate company called Bayrock. He set up offices in Trump Tower, two floors below Mr Trump.

"I decided to stop and knock on his door and tell him that we should start doing business together. I walked in and told him I'm going to be the biggest developer in New York City and you should get in now before it's too late," he said.

"He laughed and introduced me to some of his staff and before you know it we started working together."

Trump Soho was marketed to Russian investors. ( Trumpsoho.ru )

Multiple bankruptcies had made it difficult for Mr Trump to secure capital for his development projects from traditional Wall Street lenders.

Sater arrived with Russian connections exactly as Mr Trump started aggressively targeting Russian customers for his real estate ventures.

"They end up pursuing three deals together. A real estate project in Phoenix that fails. A project in Florida that fails. Then ultimately the Trump Soho in Manhattan, which opens to great fanfare but also ultimately fails," O'Brien said.

It's these deals, in particular Trump Soho, and allegations of laundered Russian money that Mr Mueller is now believed to be looking at closely.

"It's clear that this is now part of Mueller's investigation. There's a clear issue as to whether or not Donald Trump had financial quid pro quos where in his business past he did favours or had relationships for people that were tied into foreign powers, who might benefit from Trump making policy changes," O'Brien said.

Trump Soho was foreclosed on in 2014. Sater denies any wrongdoing.

Donald Trump has said he barely knows Felix Sater. ( Supplied: Felix Sater )

Despite their obvious close working relationship, on multiple occasions Mr Trump has said he barely knows Sater. In a sworn deposition in 2013, the future President said "if he [Sater] was sitting in the room right now I really wouldn't know what he looked like".

When Four Corners questioned Sater about Mr Trump's refusal to acknowledge their business relationship, he said the subject made him uncomfortable.

"You should ask him. I don't speak for him," Sater said.

O'Brien said the US President simply doesn't want to admit what he knows about Sater.

"It raises the question of why Donald Trump as a business person would get into bed with partners like that. I don't think he thought a lot of these things would ever come to light," he said.

Diplomacy and development in Moscow

Sater continued working with Mr Trump, including during the presidential campaign in 2015, when he and Mr Trump's personal lawyer, Michael Cohen, began work on new plans for a Trump Tower in Moscow.

In a November email to Mr Cohen, Sater wrote "… buddy, our boy can become president of the USA and we can engineer it. I will get all of Putin's team to buy in on this."

In late 2015 Mr Trump signed a letter of intent outlining the terms of the licensing agreement for Trump Tower Moscow.

Sater went so far as to try to get the then US presidential candidate to Russia to meet with Vladimir Putin in Moscow for the building's launch.

"We were hoping that he would go for the ribbon cutting of the project, and we would try to get both him and Putin on stage for the ribbon cutting," he said.

In the end, the deal went nowhere. Pressure was growing on the Trump campaign over his overseas business deals and Mr Trump publicly distanced himself from any association with Russia.

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So where has this left Sater? A man who has been many things in his 52 years: a stockbroker, a criminal, a real estate developer, an intelligence asset and a hunter of terrorists.

For now, he remains at the centre of the biggest story of the century, with Mr Mueller circling.

"It feels stranger than anything you can ever imagine. It's just wild. You wake up in the morning, you look in the mirror and say, 'Wow, is this really happening?'"

"All I wanted to do was build the tallest building in Europe and make a tonne of money. If I could have gotten Trump and Putin on the same stage at the ribbon cutting, what a home run.

"How was I supposed to know that those emails [to Michael Cohen] would put me into the biggest political controversy since Watergate."

For the record, Sater said he doesn't believe Mr Trump colluded with any foreign powers.

"I just wish that the investigation concludes properly, honestly, fairly, after everything has been figured out. It's my hope that somebody that I know and worked with did not collude and certainly I hope that that's the result," he said.

Watch part one of Trump/Russia: Follow the Money at 8.30pm AEST on ABC TV and ABC iview.