Hundreds of pages of documents partially declassified by the Justice department on Friday reveal that the FBI was conducting an investigation of overseas CIA prisons.

The documents were released as part of a lawsuit filed by the American Civil Liberties Union and Judicial Watch, a Washington-based advocacy group. Many of them were previously released but some of the censoring has been removed.

In September of 2002, FBI officials visiting an overseas prison run by the CIA found prisoners 'manacled to the ceiling and subjected to blaring music around the clock', according to the documents.

Handwritten notes attributed to Justice Department officials discuss the possibility of prosecuting CIA employees. Senior FBI officials questioned the legality and effectiveness of the CIA's interrogation methods and prison conditions. An interrogation involving threats with a gun and power drill was the focus of discussion in the notes, but Justice Department officials eventually declined to prosecute the CIA official.

A 2008 report details the FBI's involvement with the interrogation of Ramzi bin al-Shiebh, one of the plotters behind 9/11. A sheet of questions were prepared for al-Shiebh with the help of the FBI, but the FBI officials "were denied direct access to him for four or five days.” When the FBI was permitted to see the detainee, he was found “naked and chained to the floor.” The FBI agent told the inspector general that he had "valuable actionable intelligence" but the CIA quickly shut down the interview, ruining the case.

Many of the pages of 'declassified' documents are heavily censored, due to DOJ restrictions.

Scott Horton, a professor at Columbia Law School, has watched the document disclosures shift the focus of a potential investigation. "Disclosures increasingly put the core of potentially criminal conduct relating to torture not with CIA agents, but rather with senior figures then at the Justice Department who were busily hushing everything up."

"The key questions here are which DOJ figures were involved in the decision not to prosecute and why did they take those decisions," according to Horton.

The September 2002 overseas visit was the last involvement the FBI had with CIA interrogations, according to the New York Times.

Some of the declassified documents can be found here.