What is an Informer?
An informer, in criminal law, a person who informs law enforcement authorities of the commission of a crime or the identity of the offender, or even tells where the offender may be found. In underworld slang, he is sometimes known as a "stool pigeon" or as a "canary," because he "sings" to the police.
Informers may be roughly divided into two categories: (1) individuals who by chance obtain useful information, as in seeing criminal conduct through a window or overhearing a conversation, and so merely provide a tip to the police on a rare occasion, and (2) persons who live on the fringes of the underworld or actually participate in me criminal activity and thus are a continuing source of information.
The first group are often motivated, as good citizens, by an interest in effective law enforcement. They usually act anonymously, but then-cooperation is sometimes encouraged by the offer of a reward. The second type of informer may, in effect, be on the payroll of the law enforcement agency. Sometimes he is accorded leniency for minor offenses if he helps catch a major offender, as when a minor drug peddler leads police to his source of supply.
Because of the value of informers in law enforcement and the need to conceal their identity, either to prolong their usefulness or to protect them from retaliation, the state is privileged not to reveal their names or whereabouts. However, the precise scope of this privilege is a matter of doubt; it may prevent a defendant from fully asserting his rights, because the informer's reliability may be crucial in deciding whether an arrest was lawfully made.