.....
But for a felony, that standard is loosened a bit. It is just reasonable and probable grounds of suspicion. Guy standing outside a convince store. Hears shots. Looks in. Cashier dead. Guy running out. He wasn't present and didn't have immediate knowledge, but obviously that is the guy that just shot the clerk. So he can tackle him and hold him until police come.
.....
Seems like that illustrates a basic problem with the law. Suppose the killer ran out the back door, and the guy you jumped on was just a customer running from the gunfire. And suppose he fights back, thinking you're the gunman's accomplice. If the citizen making his "arrest" -- maybe forcibly -- didn't actually observe a crime, he can't really be sure what happened.