The letter did not define any of the circumstances.

Mr. Leahy said in a statement late Tuesday night that the letter “echoes what other administration officials have said about the use of waterboarding” but that it did not “answer the critical questions we have been asking about its legality.” He said that Mr. Mukasey “knows that this will not end the matter” and that he can expect “to be asked serious questions at the hearing tomorrow.”

The Bush administration has confirmed that the Central Intelligence Agency used waterboarding against a small number of Qaeda figures captured after the terror attacks of Sept. 11, 2001. The administration has said waterboarding was stopped several years ago in the wake of protests over the practice, in which suspects are placed on a flat surface, cloth or cellophane is put over their faces, and water is then poured over them.

The question of whether waterboarding amounts to torture nearly derailed Mr. Mukasey’s nomination for attorney general. At his Senate confirmation hearings in October, he refused to say whether he considered the technique to be torture or to be otherwise illegal. He said he needed to withhold judgment until he had received classified briefings on the subject if confirmed.

Several Democratic senators said then that his refusal to define waterboarding as torture had led them to oppose confirmation. He was confirmed on a vote of 53 to 40, and the 13-vote margin was the narrowest for a nominee to the post in more than 50 years.

Mr. Leahy and the nine other Democratic members of the Judiciary Committee wrote to Mr. Mukasey last week to insist again that he answer the question of whether waterboarding was torture. The attorney general suggested in comments to reporters at a news conference last Friday that he might never feel compelled to answer the question, no matter how often it was asked by lawmakers and the press.