PREWAR INTELLIGENCE WASHINGTON, July 30 - A senior leader of Al Qaeda 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...