The man died of a heart attack. Dr. Bronson has been the staff physician here for fifteen years and I assure you that when he pronounced him dead, he was dead.
12 seconds sound clip from the The Adventures of Superman - The Mystery of The Walking Dead classic radio series.
You can hear this line at 00:18:19 in the radio play.
Quote context
[...]
- But that's impossible. At ten twenty-five he was in his cell - either dead or alive.
- How? Since when do dead men talk over the telephone?
- Well, assuming he wasn't dead - that he was in a - a cataleptic trance...
- Oh, no.
- I'm sorry he's not on duty now so he could tell it to you himself.
- Oh, now don't misunderstand me. I'm not questioning it for a moment. It's just that...
- Well, there doesn't seem to be any other explanation. Unless you believe in the supernatural.
- If I were you, I'd forget about it.
- Miss Lane is in a state of collapse, and frankly, I'm getting worried.
[...]