In 1978, Ed Cantrell decided to talk with Rosa. Cantrell and two other officers pulled up in a car outside a lounge on Elk Street and Rosa came out wearing a holstered gun and carrying a glass of wine. Rosa got into the backseat of the car; Cantrell was in the passenger front seat. At some point in their conversation, Rosa called Cantrell a “MF” and arched back as if he was going for his gun; his hands were allegedly free, the glass of wine held between his legs. A shot rang out, and Rosa laid dead in the backseat, a bullet wound from Cantrell's gun between Rosa's eyes. Rosa's gun was still holstered; his hands around the wine glass.

Ed Cantrell was charged with first-degree murder and the case drew national media attention. Cantrell's initial lawyer, Robert E. Pfister, brought in the now legendary, Gerry Spence, for the defense. An overwhelming problem for the defense was how Cantrell, who was seated in the front seat, could get his weapon out of his holster, and shoot Rosa if in fact Rosa was going for his gun. Even Spence, in his book Gunning for Justice confessed that he initially believed that Cantrell was an "assassin."

Trail lawyers frequently save their best witness for last and will occasionally make a "show." Juries remember best that which they heard first, and they heard last. Spence called as his last witness a former border patrolman, National Rifle Association shooter, and author of the book No Second Place Winner, Bill Jordan.

Spence received permission for a courtroom demonstration and gave Jordan a holstered gun loaded with blank rounds. A young deputy was given a cocked gun loaded with blank rounds and was told to point the gun at Jordan and if Jordan made any move towards his gun to pull the trigger. All the deputy had to do was pull the trigger.

All of a sudden, a shot rang out. The deputy stood chagrinned, with his weapon still unfired. Jordan testified that the normal human reaction time is 0.50 of a second second, but he had trained himself to draw and fire in 0.27 of a second, nearly half the time for someone else merely to recognize that an opponent is going for his gun.

Jordan testified that he knew Cantrell and that they has been shooting together. Spence asked Jordan “You are certainly fast on the draw. What about Sheriff Cantrell?" Jordan answered, "Ed? Well, I reckon he's a mite bit faster'n me." The jury found Ed Cantrell not guilty.

Under Wyoming law, as it is in most states, it is not necessary that Rosa actually be going for his gun, only that Cantrell reasonably believed that he was. Thus, even if Rosa were merely going for his wine glass, if Cantrell reasonably believed that Rosa was going for his gun, Cantrell was not guilty As noted by the Wyoming Supreme Court in Hernandez v. State, 976 P. 2d 672 (Wyo. 1999):

The right of self-defense exists whether the threatened danger is real or apparent and that it is not necessary for the danger to be real or impending and immediate before self-defense is justified as long as the defendant reasonably believes it is.

Even though Cantrell was found not guilty, this is not always the case in situation such as this. It is possible to be so fast in your defensive actions that you become the attacker. Sometimes it may be a fine line between you being the victim and you being the attacker. For it to be a self-defense situation, the law requires you to believe you are in imminent danger before you may act in self-defense. This does not mean you have to wait until the attacker actually attacks you, but if you react so quickly that witnesses say you attacked before the other person had made any movement, you may be found guilty of a crime.