rch arm of Congress, was the first official assessment of a question that has gripped Washington for three weeks: Did Mr. Bush act within the law when he ordered the National Security Agency, the country's most secretive spy agency, to eavesdrop on some Americans?

The report, requested by several members of Congress, reached no bottom-line conclusions on the legality of the program, in part because it said so many details remained classified. But it raised numerous doubts about the power to bypass Congress in ordering such operations, saying the legal rationale "does not seem to be as well grounded" as the administration's lawyers have argued.

The administration quickly disputed several conclusions in the report.

The report was particularly critical of a central administration justification for the program, that Congress had effectively approved such eavesdropping soon after the Sept. 11, 2001, attacks by authorizing "all necessary and appropriate force" against the terrorist groups responsible. Congress "does not appear to have authorized or acquiesced in such surveillance," the report said, adding that the administration reading of some provisions of federal wiretap law could render them "meaningless."

The president acknowledged last month that he had given the security agency the power to eavesdrop on the international telephone and e-mail communications of Americans and others in the United States without a warrant if they are suspected of ties to Al Qaeda.

The Justice Department is investigating the disclosure of the program, first reported in The New York Times. With Congressional hearings expected this month, the Congressional research report intensified debate on the program. Administration lawyers quickly responded that Mr. Bush had acted within his constitutional and statutory powers.

"The president has made clear that he will use his constitutional and statutory authorities to protect the American people from further terrorist attacks," said Brian Roehrkasse, a Justice Department spokesman, adding that the program represented "a critical tool in the war on terror that saves lives and protects civil liberties at the same time."

Many Democrats and some Republicans pointed to the findings as perhaps the strongest indication that Mr. Bush might have exceeded his authority in fighting terrorism.

Representative George Miller, Democrat of California, who leads the House Democratic Policy Committee, said the report "raises serious questions about the president's legal authority to conduct domestic spying."

Mr. Miller said the justifications for the program were unacceptable.

Senator Dianne Feinstein, Democrat of California, said the report made "absolutely clear that the legal authorities advanced by the president in justifying domestic surveillance are on very shaky ground."

Thomas H. Kean, a Republican who was chairman of the Sept. 11 commission, weighed in for the first time in the debate. Mr. Kean said he counted himself among those who doubted the legality of the program. He said in an interview that the administration did not inform his commission about the program and that he wished it had.

The Foreign Intelligence Surveillance Act, which Congress passed in 1978 after widespread abuses by intelligence agencies, created a system for court-ordered wiretaps for terrorism and espionage suspects. That system "gives very broad powers to the president and, except in very rare circumstances, in my view ought to be used," Mr. Kean said.

"We live by a system of checks and balances," he said. "And I think we ought to continue to live by a system of checks and balances."

One reason the administration has cited for not seeking to change the intelligence law and obtain specific approvals for eavesdropping was that it might "tip off" terrorists to the program. The Congressional research service found that unconvincing.

"No legal precedent appears to have been presented," the study said, "that would support the president's authority to bypass the statutory route when legislation is required" simply because of secrecy.

Opinions on domestic spying have largely broken down, though not exclusively, along partisan lines, causing splits between the top Republicans and Democrats on the House and Senate Intelligence Committees.

The analyses of the Congressional Research Service, part of the Library of Congress created in 1914, are generally seen as objective and without partisan taint, said Eleanor Hill, staff director of the Congressional inquiry on the Sept. 11 attacks.

Because of its importance, the report was repeatedly reviewed by senior staff members at the research service for accuracy and bias before its release, officials there said.

Some Democrats say the administration bypassed the authority of Congress in ordering the eavesdropping. One congressman said he was actively misled. In a letter released Friday, Representative Rush D. Holt, a New Jersey Democrat on the Intelligence Committee, complained to the N.S.A. over what he described as deception by its director, Lt. Gen. Keith B. Alexander of the Army.

Mr. Holt, a physicist who has worked as an arms control specialist at the State Department, visited the agency on Dec. 6 for a briefing by General Alexander and agency lawyers about protecting Americans' privacy. The officials assured him, Mr. Holt said, that the agency singled out Americans for eavesdropping only under warrants from the Foreign Intelligence Surveillance Court.

After the program was disclosed, Mr. Holt wrote a blistering letter to General Alexander, expressing "considerable anger" over being misled. An agency spokesman, Don Weber, declined to comment on the letter.

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