WASHINGTON — During his confirmation last year, Attorney General Eric H. Holder Jr. failed to notify the Senate that he had signed several briefs urging courts to reject President George W. Bush’s claim that he had the power to imprison an American citizen as an “enemy combatant,” the Justice Department acknowledged Thursday.

“The briefs should have been disclosed as part of the confirmation process,” said Matthew Miller, a Justice Department spokesman. “In preparing thousands of pages for submission, it was unfortunately and inadvertently missed. In any event, the attorney general has publicly discussed his positions on detention policy on many occasions, including at his confirmation hearing.”

But at a meeting of the Senate Judiciary Committee, which held the confirmation hearing, Republicans signaled that they were likely to attack Mr. Holder over his joining the briefs — and his failure to list them, along with other public documents, on a routine confirmation questionnaire — when he testifies before them later this month.

“Are we expected to believe that then-nominee Holder, with only a handful of Supreme Court briefs to his name, forgot about his role in one of this country’s most publicized terrorism cases?” asked Senator Jon Kyl, Republican of Arizona.