In a development that Washington calls "extraordinarily significant," Pakistan arrested a man Saturday suspected of being one of the masterminds behind the Sept. 11 attacks.

Khalid Shaikh Mohammed, on the FBI's most wanted list since the fall of 2001, was picked up during a raid on a house in Rawalpindi, a city near Islamabad, officials said. Two other people accused of being members of the al-Qaeda network were also detained.

"The United States commends Pakistani and U.S. authorities on the completion of a successful joint operation, which resulted in the detention of several al-Qaeda operatives, including Khalid Shaikh Mohammed," White House spokesman Ari Fleischer said in a statement.

Khalid Shaikh Mohammed (FBI photo)
Khalid Shaikh Mohammed (FBI photo)

Mohammed, 37, was one of Osama bin Laden's "most senior and significant lieutenants, a key al-Qaeda planner, and the mastermind of the Sept. 11 attacks," according to Fleischer.

Pakistani officials said Mohammed would probably be extradited to the United States, but U.S. authorities suggested he might be interrogated in an undisclosed foreign country first. The White House said it was too soon to determine if the accused would be tried by a military tribunal.

U.S. President George W. Bush is reportedly thrilled by Saturday's news of the arrest.

"It's hard to overstate how significant this is," Fleischer said. "It's a wonderful blow to inflict on al-Qaeda."

Mohammed, born in Kuwait, is a Pakistani national who lived near the border with Afghanistan. In addition to being accused of planning the Sept. 11 attacks, he is also linked to last year's deadly bombing of a synagogue in Tunisia.

He is also the uncle of Ramzi Yousef, who was convicted in the 1993 bombing of the World Trade Center.

The FBI had offered a reward of up to $25 million for information leading to the arrest of Mohammed. American agents and Pakistani authorities were both involved in Saturday's operation, officials said.

A Canadian expert on terrorist groups said Saturday's capture proves that the U.S. has spies deep inside al-Qaeda. Martin Rudner said it could be the first in a series of raids, and might lead to the eventual capture of Osama bin Laden.