Here's a prime bit of reporting from an article on ksl.com, referring to a man found murdered:
"In fact there are no witnesses and therefore no suspects."
So I gather two things from this: the only possible suspects are people who are witnesses, and all witnesses are instantly suspects. "Well, that wraps up THIS case! We don't have any witnesses, so we don't have any suspects. Too bad someone wasn't watching out the window when the crime was committed, so we could have suspected them."