Their partnership was cut short by the arrest of Mr. Fruman and Mr. Parnas, who were accused of misrepresenting the source of a $325,000 donation to a pro-Trump fund-raising committee.
The donation had been made under the name of an energy company, although that was not the true source of the contribution, prosecutors said. Along with two others, the men were also charged with soliciting the Russian campaign contributions for marijuana licenses.
In September 2020, the prosecutors announced a new round of charges, some of them involving a business Mr. Parnas had started, known as Fraud Guarantee. Mr. Fruman was not connected to that company, but Mr. Parnas had worked on it closely with another associate who had been accused in the marijuana license donations, David Correia.
Mr. Correia pleaded guilty in October to, among other things, duping investors in Fraud Guarantee, which was supposed to offer a kind of insurance against financial swindles. He was sentenced to a year and a day in prison.
In recent months, investigators have appeared to train their focus on Mr. Giuliani. The investigation reached a turning point in April, when the F.B.I. searched Mr. Giuliani’s home and office and seized his electronic devices, in part seeking evidence about his effort to oust the ambassador.
Mr. Giuliani has not been accused of wrongdoing and has said he was acting on behalf of Mr. Trump, not the Ukrainians. He has denounced the F.B.I.’s seizure of his electronic devices as a “corrupt double standard” by the Justice Department, which he said had ignored “blatant crimes” by the Bidens.
Benjamin Weiser contributed reporting.