Lester Holt

On the form that asked for cause of death, Dr. Lukasiewicz wrote, pending, no definitive answer.

But in a few days' time, he would have more information.

The blood work was back from the lab.

Autumn Klein had suffered a very unnatural death.

Coming up... What exactly had killed Autumn?

Or is the question, who?

When Dateline continues...

Two days after Allegheny County's associate medical examiner performed his autopsy on Autumn Klein, the phone rang.

The voice on the other end was from the hospital.

Autumn's blood tests were back.

Dr. Todd Lukasevic was startled to hear what the lab found.

Lethal, deadly amount of cyanide.

What did that tell you?

That told me that I have a cause of death now.

Cyanide, the poison of the Nazi death camps, and the Jonestown massacre of the 70s.

Lethal, fast-killing stuff.

Not a common cause of death.

I have done approximately 3,500 cases in my career, and this is my first case of cyanide poisoning.

The lab's toxicology work found cyanide levels of 3.35 milligrams per liter in Autumn's blood.

So this is a lot of cyanide.