U.S. officials hailed the arrest of Khalid Shaikh Mohammed as the most significant development in the ongoing war against Osama bin Laden's terrorist network.
"It's hard to overstate how significant this is," White House spokesman Ari Fleischer said. "It's a wonderful blow to inflict on al-Qaida."
Pakistani officials also claimed victory in the Saturday arrests.
"It was the work of Pakistani intelligence agencies... It is a big achievement. He is the kingpin of al-Qaida," Rashid Qureshi, spokesman for Pakistani President Pervez Musharraf, told Reuters.
Mohammed, who was born in Kuwait, but later became a Pakistani citizen, is considered by many U.S. authorities to be the third-ranking al-Qaida leader, behind bin Laden and Egyptian Ayman al-Zawahiri.
"Khalid Shaikh Mohammed is one of Osama Bin Laden's most senior and significant lieutenants, a key Al Qaida planner and the mastermind of the September 11th attacks, Fleischer said in an earlier statement. "He is known to have been centrally involved in plotting by Al Qaida terrorists since September 11th, 2001--including plots to launch attacks within the United States."
Mohammed was placed on the FBI's "Most Wanted Terrorist" list when it was unveiled in October 2001. On the list, the FBI make no mention of the Sept. 11 attacks, but say the 37-year-old is sought for an earlier terror plot that focused on the bombing of American civilian airliners.
"Khalid Shaikh Mohammed is wanted for his alleged involvement in a conspiracy plot, based in Manila, The Philippines, to bomb commercial United States airliners flying routes to the United States from Southeast Asia in January of 1995," the FBI Web site reads. "He was indicted in the Southern District of New York in January of 1996."
American officials said Mohammed also had a hand in many of al-Qaida's other major operations, including the 1998 bombing of two U.S. embassies that killed more than 200, the attack on the USS Cole that killed 17 Americans sailors and a bombing of a synagogue in April that killed 19.
Mohammed has strong family ties to several other radical Islamic militants. He is the uncle of convicted 1993 World Trade Center conspirator Ramzi Yousef, one of his brothers also belongs to al-Qaida and another died in Pakistan when a bomb he was making exploded.
Reports indicated that Mohammed was likely to be turned over to the U.S. quickly, but an American official told Reuters it was "unlikely" he would to be legally extradited to the United States. Instead, Reuters quoted the officials as saying, "he will probably go the route of other top al-Qaida leaders like Abu Zubaida," a terrorist financier who was captured in Pakistan, but has been interrogated by U.S. authorities at an undisclosed location overseas.