The PCA investigation into complaints made by a child-abuse campaigner against Humberside Police was also hampered because the Crown Prosecution Service took three years to release documents.
The inquiry started in 1992 after Dianne Core, who runs a Humberside group, Childwatch, complained about the way police had conducted an investigation into claims of sexual abuse made by a number of children.
She alleged that a senior officer had been "rude and unsympathetic" to children and parents, and officers were "involved in suppressing information about paedophile activity in order to `protect top people'."
The PCA ordered a full second investigation, headed by Chief Superintendent Joe McNally, of South Yorkshire Police.
But the final reports, supported by 654 statements and 2,300 other documents, were only completed this month - four years after the investigation started, and nine years after the original claims of sexual abuse were made.
An interim statement on the inquiry by the PCA's deputy investigation chairman, John Cartwright, said: "The task of reconstructing and reinvestigating the original inquiry has meant that the complaint investigation has taken four years to complete. A significant constraint on the thoroughness of the investigation has been the refusal by a number of officers and ex- officers to be interviewed."
The Deputy Chief Constable of Humberside Police, Allan Charlesworth, said that of 27 officers who were asked for interviews, only 15 agreed.
But he said: "While I'm aware that the refusal of officers to be interviewed did not make the task of the investigating officer any easier, we must be careful not to condemn them simply for exercising one of the fundamental rights of justice - the right to remain silent."
A CPS spokeswoman said: "The delay in obtaining documents from the CPS was not in any way designed to be obstructive to the investigation. The file was misplaced but as soon as it came to light it was made available."Reuse content