Senior officials of the Bush and Clinton administrations began testifying before the National Commission on Terrorist Attacks Upon the United States looking into the Sept. 11 terrorist attacks.

The secretaries of defence and secretaries of state from the two administrations testified publicly about what they knew of the growing threat of terrorism against the U.S. in the past decade, and what they did about it.

Madeleine Albright, Bill Clinton's secretary of state, said fighting terrorism had been a priority and the administration was prepared to act.

Madeleine Albright

"On the USS Cole [attack in Yemen], we were obviously prepared to respond, but we did not have definitive evidence that it really was committed by Osama bin Laden and al-Qaeda. That evidence came after we were out of office. But had we had definitive evidence, I can assure you that we were prepared to act militarily," she said.

Earlier this week, Richard Clarke, a retired top counter-terrorism expert in four White House administrations, charged in a new book that he had repeatedly warned incoming Bush officials, including Secretary of State Colin Powell, of the threat from al-Qaeda.

Powell was grilled as to why in its early days in power, the Bush administration didn't act more quickly against bin Laden and al-Qaeda. Powell's answer was that the U.S. likely couldn't have stopped the Sept. 11 attacks.

Colin Powell

"Anything we might have done against al-Qaeda during this period, against Osama bin Laden, may or may not have had any influence on these people who were already in the country, already had their instructions, had already burrowed in and were getting ready to commit the crimes that we saw on 9/11."

Defence Secretary Donald Rumsfeld held out the possibility that another attack will be attempted. "We can't know where or when, or by what technique. That reality drives those of us in government to ask the tough questions: when and how might that attack be attempted? And what will we need to have done today and every day before the attack to prepare for it and, if possible, prevent it?"

While his highest foreign policy officials testified on Capitol Hill, at the White House, President George W. Bush rejected charges his administration had been slow to act against a growing terrorist threat. "Had my administration had any indication that terrorists were going to attack New York City on September 11, we'd have acted," he said.

The hearings will continue with testimony from Richard Clarke whose new book is so critical of Bush and his officials.