She was a wife. She was a mother.

She was the most amazing woman I've ever met in my life.

And then suddenly, one day she was gone, poison, my carbon monoxide.

He's like, I couldn't get a pulse. I couldn't get a pulse. I couldn't get a pulse.

Her husband blamed a faulty water heater.

Police blamed him.

You don't think this is an accident.

No. There was just one problem, and it was a big one.

No one could figure out exactly how he might have done it.

He's an expert in gases. He's an asthesiologist.

Did you kill your wife, Kathy?

Thanks for joining us. I'm Lester Holt.

Hundreds of Americans die every year from accidental carbon monoxide poisoning.

But according to police in Ohio, one of those deaths wasn't an accident.

It was murder.

And they say if anyone would know how to use a deadly gas to murder someone,

it was their prime suspect.

Here's Kate Snow.

He's come back to where it all began.

To the town, the streets, he knew so well.

Remembering the old times when life was simple, love was young,

and the future held so much promise.

...