Police dog

A police dog, known in some English-speaking countries as a "K-9" or "K9" (a homophone of "canine"), is a dog that is specifically trained to assist police and other law-enforcement personnel in their work. Their duties include searching for drugs and explosives, searching for lost people, looking for crime scene evidence, and protecting their handlers. Police dogs must remember several hand and verbal commands. The most commonly used breed is the German Shepherd.

In many common law jurisdictions, the intentional injuring or killing of a police dog is a felony.

Dogs have been used for law enforcement since at least the Middle Ages. Money was then set aside in the villages for the upkeep of the parish constable's bloodhounds that were used for hunting down outlaws. In France, dogs were used in the 14th century in St. Malo. Bloodhounds used in Scotland were known as "Slough dogs" - the word "Sleuth", (meaning detective) was derived from this.

The rapid urbanization of London in the 19th century increased public concern regarding growing lawlessness - a problem that was far too great to be dealt with by the existing law enforcement of the time. As a result, private associations were formed to help combat crime. Night watchmen were employed to guard premises with many of these individuals provided with firearms and dogs to protect themselves from the criminal elements.

One of the first real attempts to use dogs to aid police in the detection of crime and the apprehension of a criminal was made in 1869 by the Commissioner of the Metropolitan Police of London, Sir Charles Warren. Warren's repeated failures at identifying and apprehending the serial killer Jack the Ripper had earned him much vilification from the press, including being denounced for not using bloodhounds to track the killer. He soon had two bloodhounds trained for the performance of a simple tracking test from the scene of another of the killer's crimes. The results were far from satisfactory, with one of the hounds biting the Commissioner and both dogs later running off, requiring a police search to find them.

...
Wikipedia