Giuliani refuses to explain where the $500,000 he got from ‘Fraud Guarantee’ came from
Rudy Giuliani appears on CBS (screen grab)

The plot thickened in the mysterious half-million-dollar payout to President Donald Trump's personal attorney.

Rudy Giuliani was paid $500,000 last year by a company founded by his Ukrainian-American business associate Lev Parnas -- but he insisted the payment was legitimate and originated in the U.S., reported the Washington Post.

“I know exactly where the money came from," Giuliani told the newspaper. "I knew it at the time."

"I will prove beyond any doubt it came from the United States of America,” he added, but did not elaborate.

Giuliani has said that he worked for Fraud Guarantee -- the Florida-based company co-founded by Parnas, who was arrested last week -- in 2018 and 2019 but did not confirm how much he was paid until the figure was reported by Reuters.

Parnas and another Giuliani associate, Igor Fruman, were arrested last week on campaign finance charges related to their work in Ukraine that prompted Trump to press that country's president to investigate Joe Biden -- which the Democratic House is now investigating as part of an impeachment inquiry.

Giuliani told the Post he was introduced to the pair by a lawyer, and he became interested in Fraud Guarantee because he had previously worked for another company that used their technology to conduct background checks.

“They were building this business with two technologies, one of which I was very familiar with," Giuliani said. "I knew the investor of it.”

Giuliani declined to name that other company, and said his firm was paid $500,000 in two installments last year.

“Some of our contracts are two or three million dollars," he said. "This was not an extraordinarily large contract."