A New York taxi operator pleaded guilty on Tuesday to improperly pocketing $5 million in state tax money and has agreed to cooperate with federal prosecutors as they investigate President Donald Trump’s personal attorney, Michael Cohen, according to a person familiar with the case.

Evgeny “Gene” Freidman, an immigrant from the former Soviet Union long known as the “taxi king” of New York, pleaded guilty to criminal tax fraud in Albany County Court. He is a former business partner of Cohen who managed taxis owned by the president’s lawyer for several years.

Freidman’s cooperation agreement could be a significant development for Cohen, whose personal business practices are under investigation by the U.S. attorney for the Southern District of New York. Special counsel Robert Mueller, who is leading the federal Russia investigation, also has scrutinized Cohen’s activities and business relationships.

Stephen Ryan, an attorney for Cohen, declined to comment, as did Brett Berman, an attorney for Freidman. Freidman’s plea and cooperation were first reported by the New York Times.

As part of the New York probe, prosecutors are investigating Cohen for possible campaign finance violations and bank fraud. Last month, FBI agents searched his home, office and hotel room. Among the documents agents sought were records related to loans he took out against his taxi medallions, people familiar with the warrant have said.

Public records show that Cohen took out a business loan in late 2014 for an unspecified amount using three taxi companies as collateral.

Investigators also sought records related to Cohen’s October 2016 payment to Stormy Daniels, an adult-film movie actress who said she had an affair with Trump, as well as other records related to Cohen’s involvement in responding to damaging news stories about Trump before his election.

Cohen entered the taxi business as a young lawyer in the 1990s, buying up valuable taxi medallions, which are required to operate a cab in New York and other cities.

By 2003, Cohen owned a fleet of 200 taxi cabs, working in partnership with a Ukrainian immigrant named Simon Garber.