Fresh claims have emerged that a key al-Qaeda suspect was waterboarded before the Bush government lawyers issued written authorisation to do so.
A former CIA agent has told the BBC that Abu Zubaydah was waterboarded by the CIA in May or June 2002.
The date was provided by former CIA agent John Kiriakou. The practice was sanctioned in written memos by Bush administration lawyers in August 2002.
The CIA says waterboarding did not take place before August 2002.
Officials have refused to tell the BBC when it did occur.
Water-boarding involves a prisoner being stretched on his back or hung upside down, having a cloth pushed into his mouth and/or plastic film placed over his face and having water poured onto his face. He gags almost immediately.
Mr Kiriakou led the CIA team that captured Abu Zubaydah in Pakistan on 28 March 2002, and was the first to speak to the badly injured captive before returning to the US.
Abu Zubaydah remains in custody in Guantanamo Bay
There he monitored the internal communications that came in (cable traffic) on Abu Zubaydah's interrogation at a secret CIA prison from the organisation's headquarters in Virginia.
Asked by the BBC's Panorama programme when the waterboarding phase of the interrogation began, Mr Kiriakou said: "That would have been at the very end of May or the very beginning of June 2002."
The key legal advice by Bush administration lawyers that deemed it acceptable was not written until August 2002.
Mr Kiriakou said he had information that by early summer, 2002, President Bush had given his written approval for the use of waterboarding.
The BBC could not corroborate Mr Kiriakou's assertions independently. The date of Abu Zubaydah's waterboarding remains a closely guarded secret.
Mr Kiriakou has been criticised in the past for downplaying the extent of the waterboarding and emphasising its efficacy.
Waterboarding had long been treated as torture under American law - and torture is illegal under both American and international law.
President George W Bush's administration have steadily maintained that they did not break the law because they received legal advice which determined that waterboarding and other harsh techniques were not torture.
Simply having a lawyer say that something is okay is not a defence, or is not much of a defence anyway
Those legal memos redefined torture very broadly to mean some extremely painful techniques could be used and still not technically constitute "torture".
Chris Anders of the American Civil Liberties Union (Aclu), said the timing is significant for those who believe that members of the Bush administration should face trial for authorising torture.
"If waterboarding was being used then, there's no one who would be able to say that they were relying on a legal opinion because there was no legal opinion at that point to rely upon," Mr Anders said.
Officials may have been given the legal go-ahead verbally prior to August, but it is not clear how much weight that would have in courts should a decision be taken to prosecute those who authorised or carried out the harsh methods.
"Simply having a lawyer say that something is okay is not a defence, or is not much of a defence anyway", said Anders.
US President Barack Obama announced in April that he would not seek to use anti-torture laws to prosecute CIA agents who relied in good faith on Bush administration legal opinions issued after the 11 September attacks.
Obama may decide to prosecute government legal advisers
Mr Obama's assurance came after the release of memos detailing those legal opinions on the range of techniques the CIA was allowed to use during the Bush administration, including waterboarding.
In his first week in office, Mr Obama banned the use of waterboarding. Rights groups have criticised the decision not to seek prosecutions.
Mr Obama has not closed the door on the possibility of prosecuting those who gave the controversial legal advice.
His administration has described waterboarding as torture.
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