Special counsel Robert Mueller's investigators have interviewed billionaire investor Tom Barrack, Jr., the Associated Press reported on Saturday.
Barrack, the CEO of Colony NorthStar, is a close friend of President Donald Trump.
The Associated Press report was based on three sources, who were granted anonymity.
One source said the questioning focused upon former campaign chairman Paul Manafort and his deputy, Rick Gates.
A second source said the questioning also included financial matters concerning the campaign, transition and inauguration.
"Barrack has rare access and insight into Trump going back decades, since their days developing real estate," the A.P. explained. "Barrack played an integral role in the 2016 campaign as a top fundraiser at a time when many other Republicans were shunning the upstart candidate. Barrack later directed Trump’s inauguration."
The Associated Press added that "few witnesses have as much insight into the president’s lengthy business career and all facets of his campaign and administration as Barrack."