Stephen J. Cina told a news conference Khan's exhumed body was too badly decomposed to determine how the poison got into it, the Chicago Tribune reported.
"Cyanide has a short half-life and may be lost over the postmortem period unless tissues are adequately preserved," Cina said. "In this case, due to advanced putrefaction of the tissues, no cyanide was detectable in the tissues or small amounts of gastric content recovered following exhumation of the body."
Khan's death had already been ruled a homicide because a lethal amount of the poison was found in his blood.
Cina said the autopsy showed one of Khan's arteries had a 75 percent blockage and heart disease may have contributed to his death. But he said "cyanide toxicity" is still the major factor.
Khan's body, following Muslim custom, was not embalmed, which Cina said complicated the autopsy.
ImTiaz Khan, the victim's brother, told the Tribune he was disappointed in the findings.
"I still hope they will [solve] it ... The whole family is frustrated now. I am not going to be quiet. I want justice for my brother," he said.
Khan, 46, died July 20, just weeks after winning $1 million in an Illinois Lottery scratch-off game. He had opted for a $425,000 lump-sum, after-tax payment.
Shabana Ansari, Khan's wife, told the Tribune in January that detectives questioned her at length, asking her what she had given her husband for his final meal.