This is from an episode of Sledge Hammer, and old TV show.
There is a woman who marries car dealers,
the day after the marriage she kills her husband, inherents all the money, then moves on to the next victim.
The autopsy shows they have no traces of poison, or alcohol, they did not have heart attacks, cancer, were not murdered with any sharp or blunt objects. did not fall to their death, or strangled or smothered.
in other words, there is no trace of how the victims were killed.
This is a really tough one. I won't put the answer up yet but I will give a couple o' hints.