The North's Police Ombudsman, Nuala O’Loan, has uncovered more than 10 RUC investigations, including several murder probes, which she believes are seriously flawed since her appointment in 1999.
Ms O'Loan’s latest concern is the unsolved murder of 16-year-old Alice McLoughlin, who was shot and killed more than a decade ago while in a police car.
The Ombudsman is launching an inquiry after she found officers failed to examine vital evidence, follow up crucial leads or pass on intelligence to investigators.
Recently Ms O’Loan issued a report containing a damning assessment of the murder inquiry into the killing of prominent GAA man Sean Brown.
The report described it as "appalling and unprofessional" and said she had uncovered "serious and unexplained errors".
This fresh criticism will put extra pressure on the British government to set up a formal inquiry into the RUC’s investigating culture.