CIA investigators credited the detention-and-interrogation programme for developing intelligence that prevented multiple attacks against Americans.

But the CIA's inspector general said it was unclear whether so-called enhanced interrogation tactics contributed to that success.

The report said measuring the success of such interrogation is "a more subjective process and not without some concern,"

Holder said on Monday he had chosen a veteran prosecutor, John Durham, to open a preliminary investigation. It aims to determine whether any CIA officers or contractors should face criminal charges for crossing the line on rough but permissible tactics.

Durham already is investigating the destruction of CIA interrogation videos.

(CCTV August 27, 2009))