The independent US commission looking into the 11 September 2001 attacks on America has begun hearing evidence on the risk that al-Qaeda posed.
The hearing will try to find whether any specific warnings were given
The current and former secretaries of state and defence are testifying.
The first witnesss - former Secretary of State Madeleine Albright - began with US efforts to "neutralise" al-Qaeda leader Osama Bin Laden.
The hearings come after ex-White House counter-terrorism aide Richard Clarke criticised the Bush policy on terror.
He accused it of ignoring the threat from al-Qaeda - concentrating instead on Iraq.
Defence Secretary Donald Rumsfeld and Secretary of State Colin Powell will defend the current administration against the wider charge that it simply did not take al-Qaeda seriously enough, says the BBC's Justin Webb in Washington.
Mr Clarke is due to testify before the commission on Wednesday.
White House spokesman Scott McClellan has accused Mr Clarke of political opportunism ahead of November's presidential poll.
Vice President Dick Cheney said that Mr Clarke clearly missed a lot of what was going on during his time at the White House.
He also questioned Mr Clarke's effectiveness in running anti-terrorism efforts, citing the attacks on the US embassies in East Africa in 1998 and other incidents.
Bin Laden chase
This is the eighth public hearing held by the bi-partisan commission, established in 2002.
The hearings - on Tuesday and Wednesday - are on "the formulation and conduct of US counter-terrorism policy, with particular emphasis on the period from the August 1998 embassy bombings [in Kenya and Tanzania] to September 11, 2001".
Former Defence Secretary William Cohen will also be testifying on Tuesday.
Commission chairman Thomas Kean said he regretted that National Security Adviser Condoleezza Rice had declined to give evidence.
ISSUES FOR COMMISSION
Congressional oversight and state of aviation security
Terrorism, al-Qaeda, and the Muslim world
Intelligence warnings against trans-national threats
Security and liberty
Border and aviation security
But they had had "extensive" private meetings with Ms Rice, who had been "co-operative", he said.
Mr Clinton's National Security Adviser Sandy Berger will also testify on Wednesday.
Central Intelligence Agency Director George Tenet will
speak on Wednesday.
A commission statement read before evidence was heard focused on US diplomatic efforts during the Clinton presidency to track down Osama Bin Laden and bring him to the US to face trial.
But the Bush administration had developed no diplomatic initiatives to deal with al-Qaeda, the statement said.