The latest details about the special counsel's interest in Cohen emerged a week after the former Trump lawyer pleaded guilty to eight federal counts, including campaign finance charges. Cohen implicated Trump in his confession about arranging payments to two women during the campaign, saying that his former boss ordered him to move ahead with the payoffs to influence the election. Cohen did not have a formal role with the Trump campaign, but he often appeared in the media as a surrogate for Trump.
The special counsel continued asking witnesses about Cohen into at least May, according to sources. In June, Ukrainian politician Andrii Artemenko, who testified before a grand jury in the Mueller probe, told ABC News that he believed Mueller's team was still interested in Cohen.
"My personal opinion is that Michael Cohen is the target of this investigation," Artemenko said in the interview. "I can't share with you the details of the questions, but from my understanding, they're keeping going with this investigation."
People who were questioned about Cohen told CNBC that they told investigators they had no knowledge of any personal business the former Trump lawyer may have done while an employee of the Trump Organization.
A spokesman for the special counsel's office declined to comment. Cohen's spokesman and attorney Lanny Davis also declined to comment.
Legal experts say Mueller's continued interest in Cohen suggests that he could still be a pivotal source of information for the larger probe into whether Russian operatives colluded with Trump campaign officials to interfere in the 2016 presidential election. Some said that the special counsel is likely trying to verify assertions being made by Trump's former attorney.
"It is too easy to poke holes in an unreliable corroborating witness who is cooperating to get as low a sentence as possible," said Nick Gravante, a white-collar defense attorney at Boies Schiller Flexner in New York. "That means as a prosecutor you have to get others to corroborate Cohen's story in order to combat any doubt being thrown toward the prosecution by defense attorneys."
There is increasing curiosity about the nature of what information Cohen could offer Mueller. In a Washington Post story published Sunday, his lawyer Davis walked back earlier claims he made to various media outlets that Cohen has information to share that Trump knew about Russia's efforts to torpedo Democratic presidential nominee Hillary Clinton. "I should have been more clear — including with you — that I could not independently confirm what happened," Davis told the Post. "I regret my error."
According to other former Trump employees, Cohen's closeness with the president could be seen as a gateway to pivotal information for investigators – particularly when it comes to the business dealings of the Trump Organization and any possible agreements it made with foreign banks.
"We know he's given testimony for a plea deal, and we don't know what Michael is going to do in connection with that," said Barbara Res, a former executive vice president at the Trump Organization who worked there for 18 years until 1996.
"In my mind, Mueller wants to see any connection Donald may have had with Russian banks" and Cohen, along with other associates, could have knowledge about that, she added.
The FBI's investigation into Cohen also has exposed another key Trump Organization employee to scrutiny. Chief Financial Officer Allen Weisselberg was granted immunity in the Cohen case.
Weisselberg was described anonymously in the Cohen indictment last week as the point person for reimbursing the former Trump attorney for expenses, including payments he made to quiet one woman who claimed to have had an affair with Trump. The White House has denied the affair.
Starting in January 2017, Weisselberg, on behalf of the Trump Organization, doled out monthly payments to Cohen that eventually totaled $420,000, according to court documents.
A spokeswoman for the Trump Organization did not return a request for comment.