A police file containing personal details of paedophiles and their victims was found dumped in the street.
The notes, containing highly sensitive pages with information identifying people in several cases, were discovered by a Hartlepool dog walker.
Cleveland Police said the matter was "clearly unacceptable" and confirmed an investigation was under way.
The force has referred itself to the Independent Office for Police Conduct (IOPC).
In a statement, Cleveland Police said: "We take information security very seriously and an investigation has been launched to determine exactly how this happened.
"This is clearly unacceptable and we have personally contacted all those directly involved.
"We have also referred the matter to the Information Commissioner's Office (ICO) and have made a voluntary referral to the IOPC."
The spokesman declined to confirm the number of cases and people involved and added no-one had been suspended.
The papers were found near a skip in Rowell Street on 20 June and the cases date back to 2016.
The IOPC said its probe into the disposal of sensitive material was "in its early stages".