US National Security Advisor Condoleezza Rice has appeared before the Commission investigating the 11 September attacks. Ms Rice told the commission that there was no silver bullet that could have prevented the attacks.
She said that the US government's response across administrations to the terrorism threat was insufficient leading up to the deadly attacks.
However, she said President Bush realised the importance of the threat from al-Qaeda as soon as he took office in January 2001, and that defeating al-Qaeda had been his ‘very first major national security policy directive’.
Ms Rice's statement follows damaging testimony by the former White House counter terrorism chief Richard Clarke.
He told the commission that President Bush ignored warnings about al Qaeda before the attacks and focused mistakenly on Iraq afterward.
Ms Rice was at the centre of decision-making on national security before the attacks, and is one of the president's closest advisors.
Earlier, Mr Bush said he is looking forward to her testimony.
The commission, made up of five democrats and five republicans, has interviewed almost 1,000 people to date.
