Glenn Simpson told the House Intelligence Committee in a private interview in November that after his firm, Fusion GPS, was hired to look into Trump, it enlisted a former British spy "to figure out what Trump's been up to" in Russian Federation "because he's gone over a bunch of times".
Simpson was hired to probe the president by a conservative website, The Washington Free Beacon.
"As we pieced together the early years of his biography, it seemed as if during the early part of his career he had connections to a lot of Italian mafia figures, and then gradually during the '90s became associated with Russian mafia figures", Simpson said.
Mr Simpson also said that in the course of carrying out research on the US President at the time he was a candidate, it had been found that the development of Doonbeg was one of several unusual deals.
FBI counterintelligence investigators have focused on the activities of Alexander Torshin, the deputy governor of Russia's central bank who is known for his close relationships with both Russian President Vladimir Putin and the NRA, the sources said.
Another Democrat on the Republican-controlled committee, Representative Jim Hines, sought to temper Schiff's comment, telling CNN that Simpson "did not provide evidence and I think that's an important point".
And, you know, because Mr Trump's companies are generally not publicly traded and don't do a lot of public disclosure, we can only...have a limited look into the financing of those projects.
"At least on paper it says it's from The Trump Organization, but it's hundreds of millions of dollars", Simpson said.
"And these golf courses are just, you know, they're sinks".
Mr Trump paid €8.7m for the luxurious Doonbeg golf resort in Co Clare, which was sold in February 2014 to Mr Trump by receivers for the property. "They don't actually make any money".
His firm examined financial statements filed for Doonbeg in the Companies Registration Office in Dublin and the NY property developer's golf courses in Scotland, he said. "So, you know, if you're familiar with Donald Trump's finances and the litigation over whether he's really a billionaire, you know, there's good reason to believe he doesn't have enough money to do this and that he would have had to have outside financial support for these things".
Simpson also said Fusion GPS did some research into Trump son-in-law Jared Kushner, but admits that Kushner is "someone who I sort of misjudged". Other elements of the dossier have been confirmed and supporters of the document say it broadly describes an observable pattern of contacts between Moscow and the Trump campaign.
"So the first thing that I would do would be to subpoena the brokers and the people, the other people that were involved in the transactions, and the title companies and the.other intermediaries that would have that kind of information", Simpson said, when asked specifically about how the committee should go about the investigating the money laundering allegations.