Even if the United States had gone after al Qaeda before the September 11, 2001, attacks it might not have prevented them from taking place, Secretary of State Colin Powell told investigators on Tuesday.
In a defence of the administration's conduct before the attacks that killed 3,000 people, Powell also said top officials mistakenly believed the main danger from the militant network was against targets abroad.
Powell's comments before a national commission examining the attacks came amid a raging election-year debate over who can better protect Americans - Republicans led by President George W. Bush or their Democratic challengers.
He said the Bush administration determined early on to destroy al Qaeda but did not complete a strategy to carry out that decision until just days before the attacks.
"Most of us still thought that the principal threat was outside the country," Powell told the commission meeting in a hearing room at the US Congress. Secretary of Defence Donald Rumsfeld was to appear later in the day.
"Anything we might have done against al Qaeda in this period or against Osama bin Laden may or may not have had any influence on these people who were already in this country, already had their instructions, were already burrowed in and were getting ready to commit the crimes that we saw on 9/11," Powell said.
Separately, a commission report said the Clinton administration had four opportunities in the late 1990s to try to kill bin Laden, the al Qaeda leader, but held back - for fear of killing innocent civilians or out of lack of confidence in the intelligence.
One CIA official told commission investigators a February 1999 sighting had presented a "lost opportunity to kill bin Laden before 9/11."
Powell's testimony followed a weekend bombshell by Richard Clarke, a former counter-terrorism official for both President George W. Bush and former President Bill Clinton, who said the Bush administration did not take the al Qaeda threat seriously before the attacks and then focused on trying to link the strikes to Iraq.
No such link has ever been made public, and many experts are sceptical that Iraqi President Saddam Hussein - a strict secularist - and the Islamic militants of bin Laden's al Qaeda organisation would ever have worked together.
CLINTON DEFENDED: In prepared remarks, Powell's predecessor, Madeleine Albright, defended the Clinton administration's record on terrorism, and said that countering the terror threat was a top priority.
Albright said she and other members of the administration would have been prepared to kill bin Laden from the time of the 1998 US Embassy bombings in Africa until the day Clinton left office.
An earlier report by commission staffers said the Bush administration had agreed on a plan one day before the attacks to combat bin Laden, which moved gradually from diplomatic pressure to military action.
The report, presented at the outset of the commission's hearings, was likely to raise more questions about Bush's election campaign assertion that he has done everything he could to protect the American people.
The report said a meeting of Bush administration officials on Sept. 10, 2001, agreed on a strategy that called first for dispatching an envoy to give the Taleban an opportunity to expel bin Laden and al Qaeda from Afghanistan.
If that failed, pressure would be applied on the Taleban through diplomacy and in the last resort the United States would seek to overthrow the Afghan rulers.
With Bush presenting himself as a "war president" for his actions against al Qaeda and his invasion of Iraq, national security has become a major issue in campaigns for the November presidential election.
Democrats say Bush gave the terrorism threat too little weight and focused too much on Iraq. Republicans say the Clinton administration did too little against the threat from al Qaeda, leaving it for Bush to deal with.
The White House has counterattacked against the allegations by Clarke and others, saying Bush recognised the al Qaeda threat and that his administration almost immediately began working on a strategy to eliminate bin Laden's network.