The arrest in Pakistan of alleged terror mastermind Khalid Sheikh Mohammed came after a series of near misses, US and Pakistani intelligence officials say.
He had narrowly eluded capture on a number of occasions - most recently just last week in the south-western city of Quetta.
KHALID SHEIKH MOHAMMED
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Information gleaned from a Middle Eastern man picked up in that raid led investigators to Rawalpindi where the alleged brains behind the 11 September 2001 attacks was detained early on Saturday, along with two others.
"At the time of that raid in Quetta the authorities were looking for Khalid Sheikh but he escaped and from there they followed him to Rawalpindi," one Pakistani official told the Associated Press.
"They got information from the man they picked up in Quetta and from phone calls until they tracked him down to Rawalpindi."
E-mail 'gave game away'
US satellite tracking of suspects' communications is believed to have played a key role in Sheikh Mohammed's capture.
The man arrested in Quetta was later identified as Mohammed Omar Abdel Rahman, the son of a blind Egyptian cleric jailed for his role in planning the 1993 bombing of the World Trade Center.
An e-mail from the younger Abdel Rahman eventually led investigators to the address in Rawalpindi, security officials say.
Occupants of the raided house, which belongs to a member of a fundamentalist Islamic party, say they were held at gunpoint by Pakistani and US agents.
Sheikh Mohammed was captured without a fight in a dawn raid
The agents took away cassettes, a computer hard drive and discs, as well as a cell phone and documents.
FBI and CIA experts have been wading furiously through the material searching for clues to possible imminent attacks.
They believe they could find names, locations and other clues to al-Qaeda cells in the United States and around the world.
Since the attacks on New York and Washington, al-Qaeda's alleged No 3 is thought to have moved between Pakistan and neighbouring Afghanistan.
Officials believe he spent much of that time in the city of Karachi in southern Pakistan, a hotbed of Islamic militancy.
They say he escaped a number of raids there.
In September last year, Karachi police identified him as the man hit by a police sniper in a shoot-out with militants.
That raid netted Ramzi Binalshibh, an alleged would-be hijacker who became a key aide to Sheikh Mohammed.
Previously, the most significant coup in the hunt for al-Qaeda suspects in Pakistan had been the arrest last March of Abu Zubaydah - al-Qaeda's suspected financier and perhaps a key link between Osama Bin Laden and the network's operation cells.
Pakistan, a key player in the US-led "war on terror", says it has arrested more than 400 suspected Islamic militants since late 2001, but Sheikh Mohammed is by far the biggest catch yet.
Some analysts have even questioned whether he was actually arrested on Saturday.
They speculate that he may have been held for some time and the news made public when it was in the interests of the United States and Pakistan.