"The route of administration of cyanide cannot be confirmed in the autopsy," Dr Stephen Cina, the Cook County Medical examiner, told reporters at a news conference here.
Cina said the autopsy revealed nothing new to help Chicago police in the investigation of the million-dollar lottery winner's cyanide poisoning death in July last year.
He said the body was badly decomposed and the autopsy could not confirm how the cyanide entered his body.
"Once again, the manner of death is cyanide toxicity. Coronary artery disease is deemed a contributory condition, and the manner of death is homicide," Cina said.
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"Cyanide has a short half-life and may be lost over the postmortem period unless tissues are adequately preserved," he said.
"In this case, due to advance putrefaction of the tissues, no cyanide was detectable in the tissues or small amounts of gastric content recovered following exhumation of the body."
The autopsy of Khan's body was conducted after his body was exhumed in January from the Rose Hill Cemetery in Chicago.
Khan had come to the US from his home in Hyderabad in 1989 and set up several dry-cleaning businesses in Chicago.
Khan's death was originally attributed to natural causes. A relative later requested the Cook County Medical Examiner take another look. After examining fluid samples, a lethal level of cyanide was found and Khan's death was declared a homicide.