After a decade on the run, the suspected mastermind of the Sept. 11 attacks, Khalid Sheikh Mohammed, was in U.S. custody on Sunday in what U.S. officials hailed as the biggest catch so far in the global war on terror. Pakistani officials said Mohammed, branded by Washington as one of al Qaeda leader Osama bin Laden's "most senior and significant lieutenants," was arrested by Pakistani agents at a house in the city of Rawalpindi before dawn on Saturday.

A Pakistani official said Mohammed was handed over to U.S. custody and taken to an undisclosed location within hours of his arrest.

It was not known if the man described by counterterrorism experts as having been behind almost every major terror attack in the last decade had been taken to Afghanistan, a military base in Cuba where other suspected al Qaeda are held, a U.S. ship or flown to the United States or a third country.

The White House said Mohammed, one of three al Qaeda suspects detained in the early morning swoop, was "a key al Qaeda planner and the mastermind of the September 11 attacks."

Officials said the others were a Pakistani and a foreign national of Arab origin. An intelligence source described the third man as an Egyptian national, but gave no other details.

However, the family of the arrested Pakistani, Ahmed Quddus, said he was the only person detained in a raid by 20 to 25 security men armed with Kalashnikov rifles on their home in Westridge, a middle-class area of Rawalpindi at 3:30 a.m. on Saturday.

Analysts describe Mohammed, a Kuwaiti in his late 30s, as a pivotal figure in al Qaeda who vetted all its recruits and who may know the whereabouts of both bin Laden and Mullah Mohammed Omar, fugitive leader of Afghanistan's former Taliban government.

Rashid Qureshi, spokesman for Pakistani President Pervez Musharraf, called the arrest a major achievement for Pakistan's security services.

The United States, under criticism for failing to arrest the top leaders of al Qaeda while focusing on a possible war on Iraq, was jubilant and claimed joint credit.

The White House said President Bush was elated.

"That's fantastic!" Bush told his national security adviser, Condoleezza Rice, who told him at 7 a.m. on Saturday that Mohammed was in custody, a White House official said.

A U.S. official said Mohammed was expected to be interrogated in an undisclosed foreign country.

Washington had put a 25 million dlrs price on his head and he was one of 22 people on the FBI's list of "most wanted terrorists."

The Sept. 11, 2001, attacks on New York and Washington by hijacked airliners killed about 3,000 people.

CELL PLANNED TO KILL POPE

Mohammed was indicted in the United States in 1996 for his alleged role in a plot to blow up 12 American civilian airliners over the Pacific and intelligence officials in the Philippines said he was also part of cell accused of plotting to kill Pope John Paul in that country in 1995.

In addition, he is suspected of involvement in the bombing of U.S. embassies in Africa in 1998 and the attack on a U.S. warship, the USS Cole, in Yemen in 2000.

And a Pakistani newspaper linked him to the kidnapping and murder of U.S. reporter Daniel Pearl, saying investigators believed Mohammed was the man who slit Pearl's throat in front of a video camera after the journalist disappeared in Karachi in January 2002 while investigating a story on Islamic extremists.

Mohammed was reported to have narrowly evaded capture last September when Karachi police identified him as a man hit by a police sniper in a shootout with militants that netted al Qaeda operative Ahmed Omar Abdel Rahman, known as Binalshibh. This was later denied by a suspected Pakistani militant.

The family of the 41-year-old Ahmed Quddus, the Pakistani arrested, said on Sunday he was mentally slow and had no connection with any extremist group.

"My brother has never been involved in any bad things," his sister Qudsia Khanum told Reuters. "Actually, he's a bit slow, he's not very clever, so I can't even begin to imagine that he could be involved with any terrorist organization.

Ahmed has a wife and two children and lived with his parents. His father is a retired microbiologist who had worked for the United Nations and had lived abroad, while his mother is a member of one of Pakistan's most prominent Islamic parties.

Pakistani Interior Minister Faisal Saleh Hayat said initial investigations since the arrests had already yielded fresh leads that could lead to new raids.

"Raids to arrest terrorism suspects are a continuous process. They have been carried out in the past and will be carried out in the future," he told Reuters.

One expert on Pakistan, who declined to be identified, said he believed Mohammed may have been held since a shootout in Karachi that netted Binalshibh.

Analysts said Mohammed could be crucial to finding bin Laden.

"Given his key position and role, it would be very surprising if he does not know the general location of Osama bin Laden," said Husain Haqqani of the Carnegie Endowment in Washington.

"Whether he will turn them in is something we don't know. The higher you are, the more devoted you are likely to be."

Mohammed was born in Kuwait, but his family is from Baluchistan, a Pakistani province bordering Afghanistan.

He is an uncle of Ramzi Ahmed Yousef, now serving a life sentence for involvement in the 1993 bombing of New York's World Trade Center, later destroyed in the Sept. 11 attacks.

Hundreds of al Qaeda members and their Taliban allies are thought to have crossed into Pakistan after U.S.-led forces began hunting for them in Afghanistan and overthrew the Taliban.

Pakistani President Musharraf told Reuters last week his security forces had since arrested 480 al Qaeda suspects.

Mohammed studied in the United States, but moved to Pakistan's northwestern city of Peshawar in the late 1980s where he and his brothers are said to have linked up with bin Laden.

PHOTO CAPTION

Image taken off the FBI 's ten most wanted website, showing two images of Khalid Shaikh Mohammed (as spelled in website), on March 1, 2003. Pakistan said it had detained a leading member of Osama bin Laden's al Qaeda network, and CNN identified him as September 11 attack probable mastermind Khalid Shaikh Mohammed. The announcement followed the detention of three people in a raid near Islamabad. (FBI via Reuters )