At the first appearance / On the face of it.
Easy way to remember
Prima = Primary (First)
Facie = Face
Before investigation, the state of the situation may be such that it came about in a normal / usual way, for which a valid case would exist. By the initial appearance of certain circumstances, without application of thought, the way any prudent person would assume it to be or have come about.
The man with a blood covered knife in his hand, next to a dead body, would be the prima facie killer, although he maybe the one who would have taken the knife out of the body to help the victim.