WASHINGTON - A senior leader of al-Qaida who was captured in Pakistan several months after the Sept. 11 terrorist attacks was the main source for intelligence, since discredited, that Iraq had provided training in chemical and biological weapons to members of the organization, according to American intelligence officials.

Intelligence officials say the detainee, Ibn al-Shaykh al-Libi, a member of Osama bin Laden's inner circle, recanted the claims sometime last year, but not before they had become the basis of statements by President Bush, Vice President Dick Cheney, Secretary of State Colin L. Powell and others about links between Iraq and al-Qaida that involved poisons, gases and other illicit weapons.

Libi, who was captured in Pakistan in December 2001, is still being held by the Central Intelligence Agency at a secret interrogation center, and American officials say his recanted claims raise new questions about the value of the information obtained from such detainees.

A report in Newsweek magazine several weeks ago first identified Libi's role in the episode. And the fact that "an al-Qaida operative" who had provided the most detailed information alleging such ties had backed away from many of his claims was mentioned by the Sept. 11 commission in a brief footnote to the report it issued this month.

The American officials say still-secret parts of the separate report by the Senate Intelligence Committee, which was released in early July, discuss the information provided by Libi in much greater detail. The Senate report questions whether some versions of intelligence reports prepared by the CIA in late 2002 and early 2003 raised sufficient questions about the reliability of Libi's claims.

Separate from the question of Libi's account, an internal CIA review of its prewar intelligence on Iraq is still under way, continuing a push to evaluate the information used as a rationale for war. The strongest White House assertions of ties between Iraq and al-Qaida that involved illicit weapons were made beginning in October 2002, when Bush said in a speech in Cincinnati that "we've learned that Iraq has trained al-Qaida members in bomb making and poisons and gases."

In the prelude to the American invasion in March 2003, those claims were echoed often by Bush and his top advisers, but they have not been repeated that allegation for at least six months.

Intelligence officials declined to say precisely when Libi changed his account, and they cautioned that they still did not know for sure which account was correct.

Both Khalid Shaikh Mohammed and Abu Zubaydah, two other high-ranking Qaida operatives in American custody, have told interrogators that al-Qaida had no substantive relationship with the Iraqi government, according to the Senate report.

Neither the Senate committee nor the Sept. 11 commission have found evidence of a collaborative relationship between Iraq and al-Qaida on any matter, much less illicit weapons, which have not been found in Iraq despite more than a year of searching.

The Senate report says that a highly classified report prepared by the CIA in September 2002 on "Iraqi Ties to Terrorism" described the claims that Iraq had provided "training in poisons and gases" to Qaida members, but that it cautioned that the information had come from "sources of varying reliability."

Most public statements by Bush and other administration officials on the matter described the assertions as matters of fact.

At the time of his capture, Libi, a Libyan, was the highest-ranking al Qaida leader in American custody. He had worked closely with Abu Zubaydah at the group's Khalden terrorist camp in Afghanistan, and was believed to have detailed knowledge of the terrorist network's plans.