Northern Irish police colluded with pro-British paramilitary groups during violence in the 1990s, a police ombudsman has said.
The report says the former Royal Ulster Constabulary officers were guilty of "collusive behaviour" in a series of murders and attempted murders.
The police force — since replaced by the Police Service of Northern Ireland (PSNI) — was also guilty of a number of investigative and intelligence failures, the ombudsman said on Tuesday.
These failings related to eight separate attacks in the 1990s during the Troubles, in which 11 people were murdered.
More than 3,000 people were killed between 1968 and 1998 in a conflict between predominantly Catholic republicans and Protestant unionists.
In a 344-page report, Ombudsman Marie Anderson has detailed the attitude of Northern Irish police to loyalist militant groups, such as the Ulster Defence Association (UDA) and the Ulster Freedom Fighters (UFF).
The police were found to have "turned a blind eye" to the use of pro-British informants and failed to warn individuals of "real and imminent threats to their safety".
Although the police may have received information that helped save lives, the use of known serious criminals as informants was "unjustifiable", the report said.
Anderson said she was "deeply concerned" about the scale and scope of the failings identified by the "long, complex and sensitive investigation".
She added that families of the victims and survivors of the attacks had raised "legitimate and justified" concerns about the police's attitude.
The PSNI has apologised to victims' families for the failings identified by the report, which they said made for "uncomfortable reading".