There is an antidote kit (PDF) available for cyanide, but doctors treating Khan would have had to suspect cyanide early in his treatment to use it.

"Because it is so rare and is used so infrequently, the medical profession may have a difficult time recognizing cyanide poisoning," Geller said.

The same is true after death. Scientists don't usually test for cyanide in the battery of toxicology tests they may run in a suspicious death and don't generally look for it unless there is a reason to suspect it.

Plus, there's an economic factor. "The harder you want to find something, the more expensive it is, and this country's forensic labs run on a shoestring budget," Geller said.

Even knowing that Khan was poisoned, it may be difficult to determine how the cyanide got there. "Cyanide does break down in the body fairly quickly, so they may not find much," Geller said.

"Now that he's been buried and embalmed, you don't have the ideal situation," said Dr. Daniel J. Spitz, a forensic pathologist and toxicologist and the chief medical examiner for Michigan's Macomb and St. Clair counties.

Spitz co-wrote the book "Medicolegal Investigations of Death," considered the bible of forensic pathology that pathologists worldwide use.

"If this were me, I'd be hoping with an autopsy every other cause of death is rejected," Spitz said. "You don't want to have a competing cause of death when you present your case in court."

In this next step in the investigation, scientists will examine Khan's brain, the liver and even solid organs to try to detect the presence of cyanide so they have more than one test showing it's what killed him -- and a better sense of how it was introduced into his body.

"Many people think, with this kind of poisoning being rare and something that may not be seen, that this would be a murder someone could get away with," Geller said. "But clearly it is not, since they did figure out this was cyanide, and there is a very good likelihood someone will get caught."