Last August, a small team of Senate investigators trying to determine how U.S. intelligence assessments of Iraq had failed went looking for answers in a place where the Bush administration believed there were not any: the offices of U.N. nuclear inspectors in Vienna. The inspectors had determined, before the war, that Iraq did not have a nuclear weapons program.

During the secret, day-long meeting at the headquarters of the International Atomic Energy Agency, the congressional sleuths focused on aluminum tubes the CIA had said Iraq was seeking to develop a nuclear weapon. It was that claim that led the CIA to conclude that Iraq was reconstituting its nuclear program.

The U.N. teams had investigated and rejected that claim, much to the anger of the White House. But others, it turned out, had rejected it, too. When the Senate investigators left Vienna that day, they took back to Washington the names of U.S. intelligence community analysts who never agreed with the CIA's claims and, in many cases, refuted them.

The information, some of which is included in the extraordinary critique of U.S. prewar intelligence efforts released Friday by the Senate Select Committee on Intelligence, reveals the extent of the CIA's determination to keep alive the Iraqi nuclear issue long after it had been thoroughly rebutted both inside and outside the agency. The report also exposed the true nature of the CIA's relationship with U.N. inspectors whose determinations about Iraq's nuclear programs ultimately prevailed.

Contrary to public statements from outgoing CIA Director George J. Tenet and other senior officials, the CIA had not provided U.N. weapons inspectors with all of the best information it had on possible weapons locations in the run-up to war, according to the report.

National security adviser Condoleezza Rice told Sen. Carl M. Levin (D-Mich.), a member of the intelligence committee, two weeks before the U.S. attacked Iraq in March 2003, that "United Nations inspectors have been briefed on every high- or medium-priority weapons of mass destruction, missile, and UAV-related site the U.S. Intelligence Community has identified."

The committee report characterized that statement and others as "factually incorrect." Of the 148 suspect sites identified by the CIA before the war, 67 were shared with the United Nations.

Not only was the CIA keeping information from the inspectors -- whose reports on Iraq's weapons would greatly influence international support for the war -- its rationale for deciding what information to share with them was "subjective, inconsistently applied and not well-documented," according to the Senate report.

The teams led by Hans Blix, director of the U.N. effort to find chemical, biological and missile programs, were stunned by how little the CIA seemed to know about suspected sites, according to a Senate source familiar with the investigation. Senate investigators interviewed Blix and the head of intelligence analysis for the U.N. inspection teams whose headquarters were in New York.

Among the details that have not surfaced in the report but were shared with Senate investigators, was that requests by the U.N. teams to interview Iraqi defectors who were providing public accounts of Iraq's weapons programs were flatly denied, according to foreign diplomats associated with the investigation. Also, nuclear inspectors were not given information on any new sites at all -- mostly because the aluminum tubes made up the extent of the CIA's nuclear case.

The CIA was convinced the tubes were to be used in centrifuges that could enrich uranium for use in a nuclear weapon. But other U.S. intelligence analysts and the IAEA produced substantial evidence that the tubes were for conventional rockets that Iraq was allowed to possess under U.N. Security Council resolutions.

The Senate report shows that when the CIA put together its intelligence on the tubes, it withheld some evidence that did not accord with its conclusions, circulated other data in ways the Senate said was "at minimum, misleading," and tried to tilt ostensibly independent consulting reports toward the conclusion that the tubes were evidence of a nascent Iraqi nuclear program.

Speaking Friday, John E. McLaughlin, the acting CIA director, said the agency's error was to write reports "without sufficient caveats and disclaimers where our knowledge was incomplete."

The bipartisan Senate report, however, depicted something more troubling: an agency that knowingly skewed its reports to fit its convictions about an Iraqi nuclear threat.

"Who could have believed that about our intelligence community, that the system could be so dishonest?" said David Albright, an expert on Iraq's nuclear establishment who has close working contacts inside the U.S. government. "People were not only not told the truth, they were given half-truths. . . . The evidence was stacked deliberately."

Much of the Senate's narrative centers on an official identified in the unclassified report only as a "centrifuge analyst" in the CIA's Center for Weapons Intelligence, Nonproliferation and Arms Control, or WINPAC, which was established to provide U.N. inspectors with information about Iraq's programs.

WINPAC analysts gave briefings to U.N. inspectors about potential weapons sites and were enormously influential because they provided their assessments to inspectors and policymakers. Among them was the centrifuge analyst, on whom the Washington Post reported last August and identified only as "Joe."

The Senate report said he was the principal author of a CIA analysis from April 10, 2001, excerpted in Friday's report, which said that the tubes "have little use other than for a uranium enrichment program" to build the core of a nuclear warhead.

That was flatly incorrect, and an Energy Department intelligence unit explained why in detail the following day in a report titled, "Iraq: High-Strength Aluminum Tube Procurement," according to the Senate report. It said the tubes were "only marginally large enough" for use in uranium enrichment and had other specifications "not consistent with a gas centrifuge end use." The rotor casing would be only one of many parts required for a centrifuge, yet "we have not seen related procurement efforts."

The Energy Department report did not identify any such rocket program specifically. But on May 9, 2001 -- much earlier than previously known -- the Energy analysts did exactly that. "Further investigation reveals," the Energy analysts said, that Iraq had bought tens of thousands of identical tubes in the 1980s and 1990s -- 900mm long, 81mm in diameter, with walls 3.3mm thick -- to build a rocket called the Nasser 81. U.N. inspectors had counted 66,737 of the tubes on the ground in 1996.

The Post reported last August that U.S. intelligence officials serving with U.N. inspectors in Iraq documented the Nasser rocket program in early 2003.

The Senate report reveals that Joe and other CIA officials knew about the Nasser rocket program nearly two years earlier.

Yet throughout 2002 and 2003, long after learning that Iraq built tens of thousands of rockets using essentially identical tubes, the agency told policymakers the tubes were not suitable for rockets and could not be intended for a rocket program.

Secretary of State Colin L. Powell, after a two-day marathon of CIA briefings, used precisely that argument before the U.N. Security Council in February 2003.

One CIA argument was that the high-strength aluminum alloy in the tubes, known as 7075-T6, was needlessly strong and expensive. Its officials did not reveal, even when asked specifically by analysts at other departments and the IAEA, that at least two NATO munitions -- the U.S. Mark 66 rocket, or Hydra, and the Italian-built Medusa -- used the same alloy. CIA officials reported numerous times that the Iraqi tubes had specifications far more precise than any U.S. rocket, another argument Powell repeated.

In fact, the Senate committee found, the Pentagon has 25 pages of specifications for its Mark 66 rocket tubes, with considerably finer tolerances.

Defense Department rocket engineers told the Senate committee that CIA analysts rebuffed them when they said the tubes resembled an Italian rocket casing. One engineer said the CIA analyst "had an agenda" and was "trying to bias us."

The tube debate continued for 18 months.

On Sept. 16, 2002, Joe sought expert support in preparation for the CIA's most extensive analysis, titled "Iraq's Hunt for Aluminum Tubes: Evidence of a Renewed Uranium Enrichment Program." He hired consultants to conduct "spin tests" on the tubes to determine whether they could withstand the extraordinary rotational speeds required for enrichment of uranium in its gaseous form.

In interviews for this story, present and former U.S. government officials with direct knowledge described details not cited in the Senate report. Joe gave the job to two engineers with ties to the Oak Ridge National Laboratory, Andrew Szady and Joseph Dooley. He instructed them to conceal their work from the Oak Ridge Field Intelligence Element, a major repository of expertise on Iraq's nuclear infrastructure.

"It was meant to be done independently," said one source involved in the events. In a single day, Joe reported, Dooley and Szady succeeded in spinning a tube to 60,000 rpm and concluded the tubes were well-suited as centrifuge rotors.

What Joe did not report was that the great majority of spin tests led to failures of the tubes. An Energy Department analysis, conducted after the CIA was twice forced to disgorge more test data, concluded that none of the tubes demonstrated sufficient strength for long-term operation in a centrifuge.

Szady and Dooley, reached at their homes, declined to be interviewed.