Federal prosecutors in Manhattan have entered into a nonprosecution agreement with Dallas law firm Jenkens & Gilchrist over its past involvement in illegal tax shelters, a scandal that has already fatally crippled the once-thriving firm.

Between 1998 and 2003, the firm’s Chicago-based tax shelter practice provided hundreds of legal opinion letters in support of tax shelters the Internal Revenue Service subsequently deemed illegal. The criminal probe of the firm by the Southern District of New York U.S. Attorney’s Office followed several civil suits by tax shelter investors whose claims the firm has agreed to settle for $85 million.