Republicans on Capitol Hill say they are "deeply concerned" over news that, during his confirmation more than a year ago, Attorney General Eric Holder failed to notify them about a terrorism-related legal brief he helped craft.

"Not only was the attorney general required to provide the brief as part of his confirmation, but the opinions expressed in it go to the heart of his responsibilities in matters of national security," Sen. Jeff Sessions, R-Ala., ranking member of the Senate Judiciary Committee who voted to confirm the attorney general, said in a statement Thursday. "This is an extremely serious matter and the attorney general will have to address it."

"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 Sen. Jon Kyl, R-Ariz. "To me that strains credulity."

A Justice Department spokesman acknowledged that the brief "should have been disclosed as part of the confirmation process," but the spokesman insisted the omission was not intentional.

"In preparing thousands of pages for submission, it was unfortunately and inadvertently missed," Justice Department spokesman Matt Miller said in a statement Wednesday, as reports of the omission began to emerge. "In any event, the attorney general has publicly discussed his positions on detention policy on many occasions, including at his confirmation hearing."

In fact, Holder discussed the issue at length during his confirmation hearings in January 2009, promising to fight terrorism "within the letter and the spirit of the Constitution." 

"Adherence to the rule of law strengthens security by depriving terrorist organizations of their prime recruiting tools," he said. "America must remain a beacon to the world. We will lead by strength. We will lead by wisdom. And we will lead by example."

In addition, during those hearings, Holder and Sen. Lindsey Graham, R-S.C., discussed future decisions over the "form to try people and how to interrogate them," as Graham put it.

Still, the "amicus brief" filed with the Supreme Court in 2004 resonates years later as Holder finds himself defending the handling of some recent terrorism cases, particularly the interrogation of alleged "Christmas Day bomber" Umar F. Abdulmutallab.

The brief -- filed by Holder, then a private attorney, former Attorney General Janet Reno and two other Clinton-era officials -- argued that the president lacks authority to hold Jose Padilla, a U.S citizen declared an "enemy combatant," indefinitely without charge.

In making their case, Holder and the others argued that using federal courts to fight terrorism, which includes providing Miranda rights to terror suspects, would not "impair" the government's ability to obtain intelligence, which they called "the primary tool for preventing terrorist attacks."

"Many terrorists who have been arrested and provided counsel have decided to cooperate and provide valuable information to the government," their brief said. "Over the last decade, the investigative, detention, and prosecutive authorities (of the federal court system) have been used in many cases not only to identify, arrest, and punish persons who have committed terrorist acts, but to disrupt and thwart terrorism before it can occur."

But the brief did acknowledge a possible risk in such use of the federal court system -- a risk, the brief said, that is outweighed by the advantages.

"It may be true that in some cases the government will not be able to obtain information from citizens who are informed of their right to counsel, or that obtaining that information may be delayed," the brief said, noting that a lower federal court characterized such a scenario as speculative. "But this is an inherent consequence of the limitation of executive power. No doubt many other steps could be taken that would increase our security, and could enable us to prevent terrorist attacks that might otherwise occur. But our nation has always been prepared to accept some risk as the price of guaranteeing that the executive does not have arbitrary power to imprison citizens."

That assertion does not reflect the same level of certainty that Holder has expressed recently about the ability of the federal court system to obtain intelligence and fight terrorism.

"I am confident that … the decision to address Mr. Abdulmutallab's actions through our criminal justice system has not, and will not, compromise our ability to obtain information needed to detect and prevent future attacks," Holder said in a Feb. 3 letter to lawmakers. "Neither advising Abdulmutallab of his Miranda rights nor granting him access to counsel prevents us from obtaining intelligence from him."

Holder recently said Abdulmutallab has been providing "very useful" information to counterterrorism officials after being persuaded to cooperate with authorities.

Two former Bush administration officials, who were the first to note Holder's omission, accused Holder of being disingenuous.

"Now that Holder is attorney general, he no longer acknowledges the risks to national security of treating terrorists as criminals," former White House Press Secretary Dana Perino and former Deputy White House Counsel Bill Burck said in a column posted on the National Review Web site Wednesday. "Holder could never admit that now, of course."

After President Obama nominated Holder to be attorney general, the Senate Judiciary Committee sent Holder a 47-page questionnaire, including a request for any briefs he had filed with the Supreme Court "in connection with your practice."

In response, Holder said he participated in a total of five such briefs, none of which dealt with terrorism-related issues. He did not include the Padilla brief, and he signed a statement saying the information he provided was accurate and complete "to the best of my knowledge."

Miller suggested on Wednesday that Holder has always been open about his views on fighting terrorism.

"The attorney general has said many times publicly (that) the government has ample lawful ability to detain and interrogate terrorists and disrupt attacks without resorting to making claims of executive power that strain the Constitution," Miller said.

In fact, in Holder's response to the Senate Judiciary Committee questionnaire, he listed a number of speeches in which he strongly condemned the Bush administration's tactics against terrorism and promised a new way forward.

"Unfortunately, in the last few years we have lost our way, with respect to our commitment to the Constitution and to the rule of law," he told a left-leaning crowd at the 2008 American Constitution Society conference in Washington, D.C., an event noted in Holder's questionnaire. "The rule of law is not, as some have seen it, an obstacle to be overcome. ... As Americans, we should bring people to justice and not hide them away from justice."

Holder's remarks from that event became the centerpiece of a recent ad from the conservative group Keep America Safe, questioning Holder's decision to hire lawyers who previously represented or advocated for detainees at Guantanamo Bay and elsewhere.