One night in October 2001, shortly after al-Qaeda's attacks on the World Trade Center and Pentagon, a private jet touched down in Karachi. Masood Anwar, a prominent Pakistani journalist, received an unexpected tip from a friend in the airport: "There were men in masks. They took a hooded man onboard in the early hours. Someone videotaped the entire thing. No one was allowed near the site."

Anwar's story, although no one knew it at the time, would be the start of a thread which led to the heart of the Central Intelligence Agency's most secret “War on Terror” operation: the "rendition, detention, interrogation" (RDI) programme, a nine-year covert effort which had scores of prisoners flown around the globe to be tortured in undisclosed sites.

The CIA started by grabbing terror suspects off the streets and transferring them in secret to interrogators in the Middle East. But soon the agency decided it needed to run its own detention facilities, or "black sites". Over the next few years, it set up a network of prisons and a fleet of private jets to move people between them.

In December 2014, the Bureau, alongside The Rendition Project, began a major project to trace the history of the RDI programme. The impetus for our investigation came from the long-awaited publication of a report into CIA torture by the US Senate Select Intelligence Committee. The authors of this report had high-level access to internal CIA documents, which they mined to produce a damning assessment of the torture programme's brutality, mismanagement and ineffectiveness. But they were compelled by the Obama administration, and by the CIA itself, to censor — "redact" — all parts of the report that could identify specific times and places where abuses had occurred.

This is important, because without being able to tie illegal activities to specific times and places, the quest for redress is hamstrung, and meaningful accountability — legal, public, historical — remains a mirage.