Straight from the FBI website:
What is the FBI’s policy on the use of informants?
The courts have recognized that the government’s use of informants is lawful and often essential to the effectiveness of properly authorized law enforcement investigations. However, use of informants to assist in the investigation of criminal activity may involve an element of deception, intrusion into the privacy of individuals, or cooperation with persons whose reliability and motivation may be open to question. Although it is legally permissible for the FBI to use informants in its investigations, special care is taken to carefully evaluate and closely supervise their use so the rights of individuals under investigation are not infringed. The FBI can only use informants consistent with specific guidelines issued by the attorney general that control the use of informants.