Deadly force

Deadly force, also known as lethal force, is the use of force that is likely to cause serious bodily injury or death to another person. In most jurisdictions, the use of deadly force is justified only under conditions of extreme necessity as a last resort, when all lesser means have failed or cannot reasonably be employed.

Firearms, bladed weapons, explosives, and vehicles are among those weapons the use of which is considered deadly force. The use of non-traditional weapons in an offensive manner, such as a baseball bat, sharp pencil, tire iron, or other, may also be considered deadly force.

The Criminal Justice and Immigration Act 2008 allows householders to use reasonable force against intruders. In certain circumstances this can be lethal force.

The United States Armed Forces defines deadly force as "Force that is likely to cause, or that a person knows or should know would create a substantial risk of causing, death or serious bodily harm or injury.". In the United States, the use of deadly force by sworn law enforcement officers is lawful when the officer reasonably believes the subject poses a significant threat of serious bodily injury or death to themselves or others. The use of deadly force by law enforcement is also lawful when used to prevent the escape of a fleeing felon when the officer believes escape would pose a significant threat of serious bodily injury or death to members of the public. Common law allowed officers to use any force necessary to effect a felony arrest, but this was narrowed in the Tennessee v. Garner ruling in 1985 when the U.S. Supreme Court said that "deadly force...may not be used unless necessary to prevent the escape, and the officer has probable cause to believe that the suspect poses a significant threat of death or serious bodily harm to the officer or others."

In the 1989 Graham v. Connor ruling, the Supreme Court expanded its definition to include the "objective reasonableness" standard—not subjective as to what the officer's intent might have been—and it must be judged from the perspective of a reasonable officer at the scene—and its calculus must embody the fact that police officers are often forced to make split-second decisions about the amount of force necessary in a particular situation.

Most law enforcement agencies establish a use of force continuum, starting with simple presence through deadly force. With this model, officers attempt to control subjects and situations with the minimum force necessary. Agencies often have policies limiting the force used to be equal or one step higher on the continuum relative to the force they are opposing.

A civilian's use of deadly force is generally justified if they reasonably believe that they or another person are in imminent danger of death or serious injury. Justification and affirmative defenses vary by state and may include certain property crimes, specific crimes against children, or the prevention of sexual assaults.

U.S. law requires an investigation whenever a person causes another person's death, but the mechanism for such investigations can vary by state. The investigation develops evidence regarding the use of deadly physical force for a particular state or jurisdiction. An investigation may be performed by a local or state police agency and also a civilian agency, such as a county prosecutor or State Attorney General. A report of the findings of such an investigation may be submitted for prosecution and made public.