The letter, from acting US attorney Lev Dassiv, also said the CIA was gathering a list of the destroyed records, any secondary accounts that described the contents and the identities of those who may have viewed or possessed the recordings before they were destroyed.
But government lawyers said some of the information, such as the names of CIA employees who viewed the tapes, may be classified.
The news came as a US justice department investigation into the tapes' destruction, begun in January 2008, is reportedly close to completion.
Eric Holder, the US attorney general, has condemned "waterboarding", in which water is poured onto a victim's masked head in order to produce a sensation of drowning, as torture.
The tapes first became an issue during the trial of Zacarias Moussaoui, a French citizen convicted of terrorism charges in 2006, after prosecutors initially said no such recordings existed of suspects being interrogated in such a manner.
They later admitted after Moussaoui's trial that two videotapes and one audiotape had been made of Abu Zubaydah, a Saudi Arabian alleged top al-Qaeda leader, and another unnamed senior al-Qaeda suspect.
Officials said the tapes were destroyed to protect the identities of US interrogators at a time when the US department of justice was debating whether the tactics used during the interrogations - which are believed to have included "waterboarding" - were illegal.
But rights groups have accused the US government of destroying the tapes in order to remove any evidence it has used the technique.
Michael Hayden, the former head of the CIA, acknowledged last year that the CIA had used "waterboarding" on three "terror" suspects, including alleged September 11 mastermind Khalid Sheikh Mohammed, Abu Zubaydah and Saudi suspect Abd al-Rahim al-Nashiri.