The US Central Intelligence Agency (CIA) has “mistakenly” deleted its only copy of a Senate report into the agency’s brutal interrogation techniques.

The CIA Office of Inspector General (OIG), the spy agency’s internal watchdog, told the Congress that the electronic copy of the Senate Intelligence Committee’s 6,700-page report and a hard disk were destroyed last summer.

A 500-page summary of the report was released to the public by then-Senate Intelligence chair Dianne Feinstein in 2014.

Feinstein said in letters to the agency and Justice Department that the CIA inspector general “has misplaced and/or accidentally destroyed” its only copy of the report.

Last August, acting inspector general Christopher Sharpley uploaded the file onto the office’s internal classified computer system and then destroyed the hard disk in what described as the standard protocol.

Meanwhile, someone else in the office misinterpreted the Justice Department’s instructions not to open the file to mean that it should be deleted from the server. Both the original and the copy were both deleted.

At some point, CIA general counsel Caroline Krass told the watchdog that the Justice Department wanted all copies of the report to be preserved. The watchdog’s officials then carried out a research to find its copy, but understood they don’t have one.

Sharpley apologized for the destruction of the report and promised to ask CIA chief John Brennan for another copy.

Cori Crider, a director with the international human rights group Reprieve, described the destruction of the report as “stunning”, saying the move was part of a bigger effort to remove the practices from history.

“One worries that no one is minding the store,” Crider said in a statement.

The report includes details about the agency’s brutal interrogation techniques such as waterboarding and sleep deprivation at prisones overseas.

The 500-page executive summary concluded that the spy agency’s interrogation methods were far more brutal than what the agency had publicly acknowledged.

The CIA employed brutal techniques like waterboarding, physical abuse, sleep deprivation, mock executions, and anal penetration performed under cover of “rehydration” to interrogate terror suspects imprisoned after the September 11 attacks.

These torture techniques migrated from the CIA’s undocumented prisons, known as black sites, to US military prisons at Guantánamo Bay, Bagram Airfield in Afghanistan, and Abu Ghraib in Iraq.

The CIA reportedly paid $81 million to the psychologists to act as contractors to help run the torture program.