A murder inquiry into the Bloody Sunday killings 40 years ago in Northern Ireland could take four years, police warned.
The probe has not started but would follow the Saville Inquiry's report, which said civil rights demonstrators shot dead by British soldiers in Londonderry at the height of the Troubles were innocent.
The Police Service of Northern Ireland has consulted prosecution lawyers as it prepares to open an investigation. That would require a team of 30 and extra specialist resources.
Chief Constable Matt Baggott said: "I do not think anywhere else in the world is facing the challenges of organised crime, paramilitary activity ... alongside having to deal with 30 years of misery in such a way."