Firing Squad. 5 Marksmen, 3 bullets. No one knows who fired the shot that killed. Done. Period. And no more criminals hanging around for 20 years longer that the actual victims of the crime were allowed.
Anyone even marginally familiar with a rifle would have no trouble at all discerning the difference between a live round and a blank. A live round was MUCH more recoil. A blank has almost none.
Two would know that they didn't fire it.