Khalid Sheikh Mohammed
say they arrested him in the town of Rawalpindi
on 1 March 2003.
The US had increased the reward for his capture to $25m.
The US authorities believe the 37-year-old Kuwaiti is a leading figure in the al-Qaeda network and helped to plan the 11 September attacks.
They accuse him of working with Ramzi Ahmed Yousef, who co-ordinated the first bombing of the World Trade Center in 1993 which killed six people.
He has also been indicted in the US on charges that he was involved in an unsuccessful attempt to blow up American airliners over the Pacific - the so-called Manila plot.
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