The botched investigation of a homicide involving a nephew of former Mayor Richard M. Daley has become part of a federal whistleblower lawsuit filed by two Chicago police officers who claim a “code of silence” permeates the Chicago Police Department.

The two cops argued in a court filing Monday that former police Cmdr. Joseph Salemme, a defendant in their case, should have to testify about his role in the reinvestigation of the death of David Koschman, who was killed by Daley’s nephew, Richard J. “R.J.” Vanecko. The city opposes jurors hearing such details.

Salemme retired last December, escaping a one-year suspension over his handling of the Koschman case five years ago.

He was one of six cops singled out by Special Prosecutor Dan K. Webb, who said he didn’t have enough evidence to convict any of them of official misconduct or obstruction of justice over the case involving Daley’s nephew, who escaped criminal charges for nearly a decade after the police twice refused to charge him. Detectives under Salemme’s command erroneously concluded in 2011 that Vanecko had punched Koschman in self-defense after including a fabricated witness statement in their final report. Salemme has denied any wrongdoing in the Koschman case.

Salemme is among a dozen former and current officers being sued by Officers Shannon Spalding and Daniel Echeverria, who claim their bosses retaliated against them for violating the code of silence by cooperating with the FBI in a criminal case involving other Chicago cops.

Webb’s report on the Koschman case “implicates Salemme as part of the cover-up, consistent with the code of silence, regarding the misconduct of the investigating officers involved in fabricating a self-defense determination for refusing to arrest Mayor Daley’s nephew for murder,” according to their motion, filed Monday.

Spalding and Echeverria sued the city four years ago after helping federal investigators convict two cops of stealing money from drug dealers. After Spalding and Echeverria returned to their jobs, they claim they were punished by their superiors, including being assigned to night shifts and spending entire shifts in a windowless room. Fellow officers told them police commanders had warned them not to respond if Spalding or Echeverria called for backup.

Mayor Rahm Emanuel might also be called to testify in their case because he has acknowledged a code of silence exists within the police department.

After Webb declined to charge any cops in the Koschman case, Emanuel and former police superintendent Garry McCarthy directed city Inspector General Joseph Ferguson to investigate the officers for potential disciplinary action.

In December, Ferguson urged acting police superintendent John Escalante to impose discipline up to and including termination for Salemme and five other cops. Escalante move to fire three officers, while Salemme and two others faced yearlong suspensions. Salemme, a 30-year veteran, decided to retire. His pension is $162,684 a year.

Koschman died in May 2004, 11 days after he was punched in the face by Daley’s nephew during a drunken argument on Division Street at Dearborn. The case remained an unsolved homicide until the Chicago Sun-Times asked to review the files in January 2011, prompting the police to order a new investigation headed by the detectives under Salemme’s command.

Webb was appointed special prosecutor in 2012. A grand jury indicted Vanecko, who pleaded guilty to manslaughter in January 2014.