Former U.S. Attorney Dan Webb has made a career of bringing tough assignments to Winston & Strawn. But none quite like this.

For the past two years, the Winston chairman has served as special prosecutor to investigate whether criminal charges should be brought in connection with a nearly decade-old homicide case. The victim, David Koschman, died after falling and hitting his head during a scuffle in a Chicago nightclub district in 2004.

Cold as the case was, it was no whodunit. Investigators knew who threw the punch that felled Koschman: Richard Vanecko, a nephew of then–Chicago Mayor Richard Daley.

No charges were filed against Vanecko at the time. In late January 2014, however, as a result of Webb’s investigation, Vanecko pleaded guilty to involuntary manslaughter.

“It’s extremely difficult to pick up a case eight-and-a-half years after the events have happened and try to unpack what happened,” says Webb’s partner Daniel Rubinstein, who with partner Stephen Senderowitz helped Webb lead the investigation.

The initial criminal inquiry ended when witnesses failed to pick Vanecko out of a police lineup. In 2011, after queries from the Chicago Sun-Times, a hasty reinvestigation followed. Despite the fact that detectives never spoke with Vanecko, they concluded that Vanecko—nearly 10 inches taller and 100 pounds heavier than Koschman—had acted in self-defense.

Koschman’s mother petitioned the Cook County Circuit Court in late 2011 for a special prosecutor, contending that Vanecko would have been prosecuted if not for his family connections. Judge Michael Toomin asked Webb to determine whether to bring charges, and to investigate whether police or prosecutors had intentionally impeded progress or suppressed evidence.

“The only way I would agree to do this,” Webb says, “was if I was given carte blanche to do everything I could to get to the bottom of it.” That meant bringing on a team of five Winston attorneys, with about 10 others assisting. The firm’s lawyers charged one-third of their 2012 rates—about $900,000 for more than 5,400 hours worked—and handled another 9,900-plus hours without pay.

Webb convened a grand jury to review evidence, holding sessions in Winston’s Chicago offices. In December 2012 the grand jury indicted Vanecko for involuntary manslaughter, and then continued to review the prior investigations. Winston’s findings filled a 162-page report that remained under seal pending Vanecko’s trial.

Instead of facing trial, though, Vanecko pleaded guilty and was sentenced to 60 days in prison, 60 days of home confinement and 30 months of probation—a penalty that Webb calls comparable to sentences in other “one-punch” cases. As part of the plea deal, Vanecko apologized to Koschman’s mother in court.

The firm’s report became public in February 2014. It detailed a pattern of lost police files and shoddy police work—although the Winston lawyers concluded that the statute of limitations for official misconduct had run out on the 2004 investigation. They also found that there wasn’t enough evidence to bring state law charges over the 2011 investigation.

That may not be the end of the story, though. According to the Sun-Times, Winston has passed its evidence to the Federal Bureau of Investigation, which may be investigating some of the same officers.

“Whatever happened, I wanted to make sure that the public understood that my office did a thorough job,” Webb says, adding, “The public’s confidence in the criminal justice system is critical.”