Fortress Investment Group LLC sued Dechert on Monday, claiming the law firm issued an “utterly false legal opinion letter” that was used by ex-attorney Marc S. Dreier, who has since been convicted of fraud, in his scheme to defraud the private equity firm.

Fortress, which manages $32 billion in assets, said in its complaint filed in Manhattan Supreme Court that Dechert failed to perform “even the most basic due diligence” and thus ended up misrepresenting itself as the independent counsel to Solow Realty & Development Company, LLC.