On Jan. 4 of last year, a local narcotics strike force conducted a raid on the Ogden, Utah, home of Matthew David Stewart at 8:40 p.m. The 12 officers were acting on a tip from Mr. Stewart's former girlfriend, who said that he was growing marijuana in his basement. Mr. Stewart awoke, naked, to the sound of a battering ram taking down his door. Thinking that he was being invaded by criminals, as he later claimed, he grabbed his 9-millimeter Beretta pistol.

The police say that they knocked and identified themselves, though...