Lawyers for Mr. Correia and the two other defendants declined to comment.
Mr. Parnas was a regular presence in Trump donor circles before the first indictment last year, which heightened scrutiny of his dealings with Mr. Giuliani. Mr. Parnas and Mr. Giuliani played a central role in a dirt-digging campaign aimed at Mr. Trump’s political rivals that led to the president’s impeachment last year.
In early 2019, Mr. Parnas, his associate, Igor Fruman, and Mr. Giuliani tried to persuade Ukraine to investigate Hunter Biden, the son of Joseph R. Biden Jr., the former vice president who is now the Democratic nominee for president.
Working for Mr. Giuliani, they also tried to dig up damaging information on Marie L. Yovanovitch, then the American ambassador to Ukraine. Mr. Trump eventually removed Ms. Yovanovitch from her post last year, a decision that was at the heart of his impeachment trial.
The investigation into Mr. Giuliani focused in part on whether he had worked to oust Ms. Yovanovitch at the behest of a Ukrainian official, which might violate laws governing lobbying on behalf of foreign governments. Mr. Giuliani has maintained that he was working for Mr. Trump, not the Ukrainian government.
Mr. Giuliani was still facing scrutiny over the summer, though the current status of the investigation was unclear on Thursday.
“To be honest, I don’t know that there ever was a Rudy Giuliani investigation,” said Robert J. Costello, a lawyer for the former New York mayor. “The fact that your name comes up doesn’t mean that you’re a target.” The prosecutors, he said, have not subpoenaed Mr. Giuliani or sought to speak with him.
On Thursday, prosecutors disclosed in a footnote in a letter to the judge presiding over the case that he had agreed to keep some information secret because their investigation is ongoing. They did not elaborate.