A private investigator who played a central role in freeing Death Row inmate Anthony Porter in 1999 filed a $25 million counter-lawsuit Wednesday saying he has since been defamed.

Paul Ciolino famously worked with Northwestern University professor David Protess and his journalism students to free Porter, who had been convicted of a 1982 double murder on the South Side.

In 1999, Ciolino and a Northwestern student traveled to Milwaukee to interview Alstory Simon, whom the Northwestern team settled on as the key suspect after their own investigation.

Ciolino later told a grand jury he obtained Simon’s videotaped confession to the murders when he showed him a video of one of Ciolino’s employees, who had posed as a witness who falsely said he saw Simon commit the shootings.

In 2014, Cook County State’s Attorney Anita Alvarez conducted an investigation that concluded Simon was railroaded, and he was freed from prison.

Last year, Simon sued Protess, Ciolino and Northwestern University in federal court in Chicago, and in March, a judge allowed the $40 million lawsuit to proceed.

On Wednesday, Ciolino filed a counterclaim against Alvarez and others, claiming they have defamed him. He said the false allegations have ruined his business and forced him to give up his detective’s license for lack of clients.

Ciolino says attorneys Terry Ekl and James Sotos, who filed the lawsuit against him, conspired with others, including former Alcohol, Tobacco, Firearms and Explosives agents and author William Crawford, a former Chicago Tribune reporter, to undo Simon’s conviction and place him, Protess and Northwestern in an unfavorable light.

“These individuals who view the hard-working advocates and the wrongly convicted as predators of a so-called ‘innocence industry’ are now brazenly claiming that Northwestern University’s Innocence Project led by professor David Protess framed [Alstory Simon] so Death Row inmate Anthony Porter could become a ‘poster boy’ for the bid to end executions in Illinois,” according to Ciolino’s counterclaim.

He says Ekl, Sotos and others participated in a documentary, “Murder in the Park,” that defamed him, Protess and Northwestern.

Alvarez did a “solid” for her “friends” Ekl and Sotos when she dismissed the case against Simon, the counterclaim says.

Ciolino says Simon had confessed five other times since his original confession in Milwaukee.

According to the lawsuit that Ekl and Sotos filed against Ciolino, Protess and Northwestern, then-Cook County State’s Attorney Dick Devine convened a grand jury in 1999 that considered Simon’s confession — as well as testimony from four witnesses who said they saw Porter at the scene of the killings — and one witness who said he saw Porter actually shoot the victims.

That grand jury didn’t indict Simon, but a second grand jury was empaneled. None of the witnesses who implicated Porter were presented to that second grand jury, which indicted Simon with the double murder, according to the lawsuit against Ciolino, Protess and Northwestern.

Simon then pleaded guilty on the advice of his attorney, Jack Rimland, who warned he would face the death penalty if he went to trial, according to the lawsuit. Rimland was a friend and associate of Ciolino, the lawsuit said.

Private eye files $25M defamation suit in Porter case