Sergeant Dennis Tueller, of the Salt Lake City, Utah Police Department wondered how quickly an attacker with a knife could cover 21 feet (6.4 m), so he timed volunteers as they raced to stab the target. He determined that it could be done in 1.5 seconds. These results were first published as an article in SWAT magazine in 1983 and in a police training video by the same title, "How Close is Too Close?"
A defender with a gun has a dilemma. If he shoots too early, he risks being charged with murder. If he waits until the attacker is definitely within striking range so there is no question about motives, he risks injury and even death. The Tueller experiments quantified a "danger zone" where an attacker presented a clear threat.
The Tueller Drill combines both parts of the original time trials by Tueller. There are several ways it can be conducted:
- The "attacker and shooter are positioned back-to-back. At the signal, the attacker sprints away from the shooter, and the shooter unholsters his gun and shoots at the target 21 feet (6.4 m) in front of him. The attacker stops as soon as the shot is fired. The shooter is successful only if his shot is good and if the runner did not cover 21 feet (6.4 m).
- A more stressful arrangement is to have the attacker begin 21 feet (6.4 m) behind the shooter and run towards the shooter. The shooter is successful only if he was able take a good shot before he is tapped on the back by the attacker.
- If the shooter is armed with only a training replica gun, a full-contact drill may be done with the attacker running towards the shooter. In this variation, the shooter should practice side-stepping the attacker while he is drawing the gun.
- Tueller, Dennis (March 1983), "How Close is Too Close?", S.W.A.T. Magazine
- Ayoob, Massad (October 1991), "Explaining the deadly force decision: the opportunity factor", Shooting Industry
- Young, Dan. "Handgun Drills, Standards, and Training Page". Retrieved 2008-04-16.