A newly declassified CIA report says interrogators threatened to kill family members of a man accused of planning the Sept. 11, 2001 attacks.

In this March 1, 2003 file photo, Khalid Sheikh Mohammed is seen shortly after his capture during a raid in Pakistan. (Associated Press)

The report, written in 2004 and released Monday by the U.S. Justice Department, said CIA officers told Khalid Sheikh Mohammed that if any other attacks happened in the United States, "we're going to kill your children."

The report also said Abd al-Nashiri, the alleged mastermind of the 2000 USS Cole warship bombing, was hooded and handcuffed and threatened with an unloaded gun and a power drill.

The unidentified interrogator also threatened Nashiri's mother and family, implying that they would be sexually abused in front of him, according to the report.

Mohammed and al-Nashiri are currently being held by the U.S. at the Guantanamo Bay detention centre in Cuba.

In another case, CIA officers staged a mock execution — firing a gun in a room adjacent to an interrogation — in an apparent effort to convince a detainee that someone nearby had been killed, the report said.

The documents released by the CIA's inspector general acknowledge that interrogators went too far with questioning techniques used after the Sept. 11 attacks on the United States under then-president George W. Bush but subsequently withdrawn by his successor, Barack Obama.

"Ten years from now, we're going to be sorry we're doing this, (but) it has to be done," one unidentified CIA officer said in the report, predicting that interrogators would someday have to appear in court to answer for such tactics.

Bush officials have denied that torture was used and defended their interrogation practices as legal.

Obama approves new anti-terrorism unit

The report detailing some of the harsh interrogation tactics was released as the U.S. government approved the creation of a new FBI-led unit to interrogate terrorism suspects.

A federal judge ordered the report be made public Monday, in response to a Freedom of Information Act lawsuit filed by the American Civil Liberties Union.

Formation of the new interrogation unit for "high-value" detainees does not mean the CIA is out of the business of questioning terror suspects, deputy White House press secretary Bill Burton told reporters.

Burton said the unit will include "all these different elements under one group" and will be located at the FBI headquarters in Washington.

The unit, to be known as the High-Value Detainee Interrogation Group, is to be led by an FBI official, with a deputy director from somewhere in the government's intelligence apparatus and members from across agencies. It will be directly supervised by the White House, but senior administration officials said the unit's agency bosses will make operational decisions.

The unit would not alter the Obama administration's decision banning harsh interrogation techniques, such as waterboarding, that were authorized by the Bush administration. The team was examining what other techniques could be used.

In a separate development, Attorney General Eric Holder has chosen veteran federal prosecutor John Durham to investigate CIA questioning of terror suspects during the Bush administration.

With files from The Associated Press