A judge in Chicago on Friday named a special prosecutor who will investigate how local officials handled the case against Jussie Smollett, who was accused of paying two acquaintances to attack him, making the assault look like a racist and homophobic hate crime.

The new special prosecutor, Dan K. Webb, is a former United States attorney and a high-profile Chicago lawyer who worked as a special counsel in the Iran-contra affair. The appointment came two months after Judge Michael P. Toomin of Cook County ordered that a lawyer be named to take another look at the case. The judge was charged with finding someone to assess whether there was any misconduct in the way the case was managed and whether there is justification for renewing the prosecution of Mr. Smollett, whose felony charges were dropped in March.

Mr. Webb told reporters on Friday that his firm would work on the case pro bono, billing Cook County for out-of-pocket expenses but not legal fees. One of Mr. Webb’s earliest tasks as special prosecutor, he said, would be filing a motion to request a special grand jury to assist him.

[A timeline of the Jussie Smollett case.]

As the United States attorney in Chicago, Mr. Webb was chief prosecutor in Operation Greylord, the undercover investigation of corrupt judges, police officers, lawyers and other public officials in Chicago. Mr. Webb gained international recognition for prosecuting Adm. John M. Poindexter, a former national security adviser to President Ronald Reagan, in the Iran-contra affair. In his current role as an executive chairman of the international law firm Winston & Strawn, he is known for defending prominent white-collar clients, including George H. Ryan, the former Illinois governor.