mother. she was the most amazing woman i had ever met in my life. and then, suddenly, one day, she was gone poisoned by carbon monoxide. he was like i couldn t get a pulse, i couldn t get a pulse. her husband blamed of faulty water heater, police blamed him. you don t think this is an accident? no. there was 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 analyses urologist. murder or an accident. did you kill your wife? hello, and welcome to dateline, 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 suspects. here is kate snow with deadly exposure. reporter: he s come back to where it all began, to the town, the streets he knew so well, remembering the old times