Mr Simpson said that based on the suspicions around whether Mr Trump is in fact a billionaire, as the Republican president has claimed to be, there was "good reason" to believe he did not have the money for his Irish and Scottish golf courses and that he would need "outside financial support".
President Trump has derided the dossier as politically motivated, and several GOP-led committees are investigating whether it formed the basis for the FBI's initial investigation into Russian election interference.
It first investigated Trump on behalf of the conservative Washington Free Beacon online news site and then for the Democratic National Committee.
He described as "fascinating" the story of Trump "financing Doonbeg in Ireland through money that we can't really trace but has sort of the fingerprints of Russian mobsters".More news: Last Slam standing: Kerber beats Sharapova in 3rd round
Simpson noted that while the statements don't "on their face" show Russian involvement, they do show "enormous amounts of capital flowing into these projects from unknown sources".
But because the Irish course and the Scottish course are under United Kingdom, you know, Anglo corporate law, they file financial statements, so we were able to get the financial statements.
The co-founder of the firm that produced the so-called Trump dossier told the House Intel Committee that early on in his investigation he saw "patterns of activity" from the Trump Organization that "might be suggestive of money laundering", including dealings with Russian ties. And these golf courses are just, you know, sinks.
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.
"And sometimes properties would be bought and sold, and they would be bought for one price and sold for a loss shortly thereafter, and it really didn't make sense to us".More news: This Text Message will 'Blow up' your Apple Gadgets
Mr Simpson said that he is mostly working off public records. A lot of what I do is analyse whether things make sense and whether they can be explained. "And that didn't make sense to me, doesn't make sense to me to this day", the former reporter said.
The firm also investigates sales of Trump-bought properties across the United States, and found several oddities in purchase patterns reminiscent of money laundering.
Steele made global headlines after it was revealed that he compiled the "Trump-Russia dossier" - an infamous cache of documents that claims the Russian government has blackmailed Trump to do its bidding after obtaining damaging information about him, including a video tape allegedly showing him engaging in "perverted sexual acts".
A spokesman for Trump International Golf Links and Hotel Ireland told the Irish Sun: "No comment".
During his testimony before the House select intelligence committee on November 14, Simpson told investigators that none of the information Steele obtained about Trump stood out as improbable.More news: YouTube Is Taking Down Those 'Dangerous' Tide Pod Challenge Videos
Donald Trump resigned as director of the family's Scottish and Irish golf courses just before his inauguration as president in January past year.
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