US President Bush has spoken about his first reaction when he saw a plane hit the World Trade Centre.
He says his initial thought was that it was a 'terrible' pilot, and he believed it must have been a 'horrible' accident.
The president says when he realised the facts, he decided there would be 'hell to pay for attacking America'.
Mr Bush was describing his reaction to what turned out to be the September 11 terrorist attacks during a town hall forum in Orlando, Florida, in response to a question from a student.
The president recalled he was visiting an primary school in Sarasota, Florida, to talk about a reading programme on the morning of the attack.
"I was sitting outside the classroom waiting to go in, and I saw an airplane hit the tower - the TV was obviously on - and I used to fly myself, and I said, 'There's one terrible pilot'. And I said, 'It must have been a horrible accident'."
Mr Bush said he didn't have much time to think about it before he sat down with a group of children.
Then his chief of staff, Andrew Card, informed him a second plane had hit the towers and America was under attack.
"I wasn't sure what to think at first," Mr Bush told the youngster. "You know, I grew up in a period of time where the idea of America being under attack never entered my mind - just like your daddy and mother's mind probably.
"And I started thinking hard in that very brief period of time about what it meant to be under attack. I knew that when I got all the facts that we were under attack, there would be hell to pay for attacking America."