Despite its limitations, an agreement with the White House that gives an independent Sept. 11 commission access to intelligence briefings for Presidents Bush and Bill Clinton is unprecedented, making an outside panel privy to some of the executive branch's most closely held secrets for the first time, according to commission officials and legal experts.

The deal -- reached Wednesday after months of tense negotiations and the threat of a subpoena -- marks a watershed moment in the long history of battles between the executive branch and outside investigators over matters of presidential secrecy and privilege, legal scholars said.

It also marks a departure for an administration that frequently has fought attempts by Congress and government investigators to review other sensitive executive branch documents. The arrangement has already prompted the Senate intelligence committee to demand similar access to White House materials related to prewar Iraq intelligence, legislative sources said last week.

"Neither we nor the White House are aware of any precedent for this in the history of the republic," said the Sept. 11 commission's executive director, Philip Zelikow. "That is true not only for our access to these items, but for many of the other kinds of access to highly sensitive materials that we have been granted."

But in securing access to the intelligence documents known as the President's Daily Brief (PDB), the commission also agreed to restrictions that are viewed as overly stringent by two Democrats on the panel and by many relatives of Sept. 11 victims.

In addition, the agreement does not resolve the question of whether substantive details from these documents or others will ever be shared with the public. The PDBs are classified, and the administration is likely to insist that they remain secret, experts said.

"One of the most important questions all along from the public standpoint is, 'What did the president know?' " said Eleanor Hill, who worked as staff director of a joint House-Senate inquiry into intelligence failures leading up to Sept. 11 that was denied access to PDBs. "I don't see any evidence at the moment that the American public is going to know the answer."

The National Commission on Terrorist Attacks Upon the United States was created in fall 2002 by Congress, after months of resistance from the Bush administration, to investigate issues related to the Sept. 11, 2001, attacks. The bipartisan commission got off to a stumbling start with debate over its membership and has since been battling the Bush administration over access to a range of sensitive documents.

The delays have cast doubts on the commission's ability to meet a statutory deadline next May. The panel has subpoenaed the Federal Aviation Administration and the Defense Department for material on the nation's air defense network. But its biggest fight was with the White House, whose resistance to granting access to the PDBs had prompted threats of a subpoena from the commission's chairman, former New Jersey governor Thomas Kean (R).

Kean said the agreement reached last week "gives the commission full access to all the documents we're asking for." A four-member review team chosen by the commission will read and take notes on PDBs that have information about al Qaeda, Osama bin Laden and other issues directly related to Sept. 11.

The review team will write summaries for distribution to the entire 10-member commission. But before they are distributed, the White House will be notified, giving administration attorneys time to object to sensitive items.

Hundreds of other PDBs will be reviewed by two team members to determine whether any relevant information has been missed. If so, the White House would have to agree to wider distribution.

Two Democratic commissioners, former senator Max Cleland (D-Ga.) and former representative Timothy J. Roemer (D-Ind.), said in interviews last week that the arrangement gives too much power to the White House to withhold information. "There are too many logjams, too many restrictions and too many reporting requirements," Roemer said.

The Family Steering Committee, a group of Sept. 11 relatives, also condemned the agreement, arguing that "all 10 commissioners should have full, unfettered and unrestricted access to all evidence" related to the attacks.

"As it now stands, a limited number of commissioners will have restricted access to a limited number of PDB documents," the group said in a statement. "This will prevent a full uncovering of the truth and is unacceptable."

But Kean, Zelikow and others said they believe the arrangement is workable and will allow the commission to learn what kind of advice was given to Bush and Clinton. Kean said that the main alternative -- to force a subpoena on the White House or CIA, which retains ownership of the PDBs -- would have guaranteed a lengthy and possibly ruinous court fight.

"A subpoena, if resisted, means you see nothing in the meantime and you go into court," Kean said. "That may take a while, and the chances are good that you would lose."

The PDB, prepared by the CIA's analytic directorate, usually consists of summaries of current intelligence threats but sometimes focuses on broader themes, sources have said. One example is an Aug. 6, 2001, PDB examining possible methods of attacks by al Qaeda, including airline hijackings, that has been publicized in news reports and congressional accounts.

Such documents have historically been held tightly, and the White House refused to give the House-Senate intelligence inquiry access to them. Elizabeth Rindskopf Parker, a former CIA general counsel who now serves as dean of the law school at the University of the Pacific in Sacramento, characterized the PDB as "sacrosanct."

"It's something you never, ever share," Parker said. "It was not something I would have regularly seen as general counsel at the CIA. It really is advising your client, the president, in the most intimate way."

Neal Katyal, a Georgetown University law professor who was a national security adviser in the Clinton Justice Department, said the agreement "is an extraordinarily broad step towards openness. . . . We're not talking about some minor cable from an embassy in Lima here. We're talking about the PDB."

At the same time, legal experts said the deal has limited use as a precedent. As a panel created by Congress, approved by the president and acting outside the boundaries of the legislative and executive branches, the Sept. 11 commission is, in Katyal's words, a "special animal."

The office of Senate Intelligence Committee chairman Pat Roberts (R-Kan.) has told the White House it expects similar access. "We've asked for equal treatment," one committee staffer said. "We've said that we, as a permanent legislative committee, would certainly hope to receive equal treatment to a temporary commission."

The White House has signaled that it may assert executive privilege, which allows the president to withhold advice and internal communications from public disclosure. The Intelligence Committee's status as a purely legislative body makes it different from the Sept. 11 panel, administration officials say.

Kean said the only similar commissions to the Sept. 11 panel were those that investigated the assassination of John F. Kennedy and the attack on Pearl Harbor.

"Our position was that what you give us doesn't set an example for Congress or for any other committee or commission," he said.