The CIA has confirmed that it destroyed at least two video tapes showing the interrogation of terror suspects.
The CIA destroyed the tapes when being scrutinised over secret prisons
According to the intelligence agency, the tapes were destroyed to protect the identity of CIA agents and because they no longer had intelligence value.
But civil liberties lawyers have refused to accept this, saying the CIA previously denied such tapes existed.
They say the move appears to be an attempt to destroy evidence that could have brought CIA agents to account.
The New York Times, which broke the story, quotes current and former government officials as saying the CIA destroyed the tapes in 2005 as it faced Congressional and legal scrutiny about its secret detention programme.
Officials feared the tapes could have raised doubts about the legality of the CIA's techniques, the newspaper says.
The tapes are thought to have shown the interrogation in 2002 of a number of terror suspects, including Abu Zubaydah, who had been a chief recruiter for the al-Qaeda network.
Water boarding: prisoner bound to a board with feet raised, and cellophane wrapped round his head. Water is poured onto his face and is said to produce a fear of drowning
Cold cell: prisoner made to stand naked in a cold, though not freezing, cell and doused with water
Standing: Prisoners stand for 40 hours and more, shackled to the floor
Belly slap: a hard slap to the stomach with an open hand. This is designed to be painful but not to cause injury
Source: ABC News
The videos were, according to the New York Times, wiped in 2005, at the time the agency was being scrutinised about its secret detention programme.
The Associated Press news agency on Thursday obtained a letter sent to all CIA employees by the agency's current director, Michael Hayden, explaining why the footage was destroyed.
In the internal memo, Gen Hayden told staff that the CIA had begun taping interrogations as an internal check in 2002 and decided to delete the videos because they lacked any "legal or internal reason" to keep them.
According to AP, the CIA chief wrote to employees: "The tapes posed a serious security risk.
"Were they ever to leak, they would permit identification of your CIA colleagues who had served in the programme, exposing them and their families to retaliation from al-Qaeda and its sympathizers."
The CIA acknowledges that these early interrogations were harsh, but Gen Hayden says that the CIA's internal watchdogs saw the tapes in 2003 and verified that the techniques used were legal.
But Senate judiciary committee chairman Patrick Leahy said the tapes' destruction was troubling.
"The damage is compounded when such actions are hidden away from accountability," he said.
The American Civil Liberties Union has accused the agency of showing an utter disregard for the law.
"The destruction of these tapes appears to be a part of an extensive, long-term pattern of misusing executive authority to insulate individuals from criminal prosecution for torture and abuse," an ACLU statement said.
The BBC's Jonathan Beale in Washington says the news is likely to trigger more questions about the interrogation techniques used by the CIA and whether they amounted to torture.
There are also questions over whether CIA agents withheld information from the courts and a presidential commission.
The CIA's failure to make the tapes available to a federal court hearing the case of the terror suspect Zacarias Moussaoui or to the 9/11 Commission could amount to obstruction of justice, according to the New York Times.
Lawyers in the Moussaoui trial and officials from the 9/11 Commission had both requested from the CIA details of any relevant interrogations of al-Qaeda suspects.
Michael Hayden wrote to all CIA employees about the tapes
After the 9/11 terrorist attacks on the US, President George W Bush authorised the use of "harsh techniques" in the interrogation of suspected terrorists.
According to our correspondent, those techniques are alleged to have included water-boarding, a method in which a suspect is held down and gagged while water is poured into his mouth in order to simulate drowning.
Human rights groups say that water-boarding - and other techniques allegedly used by the CIA - can be defined as torture under various international treaties to which the US is a signatory.
The Bush administration has always maintained that it does not allow the use of torture.