The capture of a top al Qaeda operative could lead to the arrest of other senior officials in the terrorist organization and leaves the group scrambling to produce new leadership to carry out future attacks, according to U.S. intelligence officials.

Khalid Sheikh Mohammed, apprehended over the weekend by Pakistani and U.S. agents, is believed to have recruited dozens of understudies over the years who could try to take over his role. Mr. Mohammed, believed to have planned the Sept. 11 attacks in the U.S. and other terror operations, himself emerged as a more powerful force inside the organization after several of Osama bin Laden's other top lieutenants were caught or killed during the U.S.-led war in Afghanistan, U.S. officials said. Until his capture, Mr. Mohammed, 37 years old, continued to plan terror operations in the Middle East, Africa and Southeast Asia, they say.

"The arrest of Khalid Sheikh Mohammed is a major counterterrorism victory, but there is no guarantee that the serious threat posed by al Qaeda will be removed," says Rodolfo Mendoza, a leading Philippine counterterrorism official who has tracked Mr. Mohammed since the terrorist operated in Manila in 1994. "Al Qaeda planners have been nurtured and trained by the leadership, and have the capacity to think, plan and execute terror plots."

Although many of Mr. bin Laden's top lieutenants have now been taken into custody or killed, a senior U.S. official estimated Sunday that roughly 60% of the organization's top operatives remain on the loose, representing dozens of militants. With each arrest, U.S. officials hope they are whittling away at al Qaeda's effectiveness. Still, officials fear that a looming U.S. invasion of Iraq could spur new attacks by al Qaeda adherents hoping to incite a broader uprising against the U.S. and its allies.

Khalid Sheikh Mohammed, the alleged Sept. 11 mastermind, is seen shortly after his capture during a raid in Pakistan Saturday in this photo obtained by the Associated Press.

The arrest of such a senior al Qaeda operative is a big win for President Bush. It undercuts criticism that he is ignoring the international war on terrorism while attempting to deal with the threat posed by Iraqi leader Saddam Hussein.

Officials plan to move Mr. Mohammed from Pakistan to an undisclosed location outside the U.S. The terror leader could face months, if not years, of interrogations. Washington has stressed that its priority isn't to prosecute terrorists, but to gather intelligence to block future attacks. Mr. Mohammed, who has described himself as al Qaeda's military leader, is believed to have operational knowledge of most of al Qaeda's activities.

Elated by the capture in a joint raid by Pakistani intelligence agents and Central Intelligence Agency personnel, some U.S. officials say that taking Mr. Mohammed out of action will do more to curtail al Qaeda attacks than catching Mr. bin Laden, who remains at large. "Some argue that he's even more important than bin Laden, because he was involved in planning ongoing attacks and bin Laden's more of the philosophical leader," says a senior U.S. official.

Whether Mr. Mohammed can help lead the U.S. to Mr. bin Laden wasn't clear. U.S. officials assume that, even though they are in hiding, al Qaeda's top leaders are still finding ways to communicate. Mr. Mohammed may know the whereabouts of Mr. bin Laden and his top deputy Ayman al-Zawahiri. Mr. Mohammed's interrogation is likely to involve security services from friendly Arab governments, in addition to trained CIA interrogators. While the U.S. doesn't divulge the techniques it uses, the lengthy process has often appeared to yield useful information, sometimes revealed accidentally under the stress of repeated questioning, officials say.

One U.S. law-enforcement official said a laptop computer was seized during Mr. Mohammed's capture. This official said it may provide useful clues about future operations and the names of other al Qaeda members.

Mr. Mohammed was caught early Saturday in Rawalpindi, a sprawling and hectic city just west of Islamabad that also serves as the Pakistan military's garrison town. He had been tracked in recent weeks by a joint team of Pakistani and CIA operatives, based on reported sightings from Pakistanis. They also made use of more-sophisticated intelligence-gathering, including phone-call traces and use of surveillance equipment that U.S. officials wouldn't describe.

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Fluent in Arabic, Persian, English and his native Urdu, Mr. Mohammed floated easily throughout the region over the last several years. He first came to the U.S.'s attention in the mid-1990s, when he was linked to a plot involving his nephew, 1993 World Trade Center bomber Ramzi Yousef, to blow up airliners over the Pacific and to fly a hijacked plane into CIA headquarters. Mr. Yousef is imprisoned in the U.S. Mr. Mohammed slipped out of U.S. hands several times over the years, including in 1996 when he left Qatar before a U.S. arrest team arrived in the country, officials say.

In recent weeks, Pakistan authorities picked up intelligence that Mr. Mohammed was shuttling between Karachi and Hub, a small town in neighboring Baluchistan province, according to one Pakistani intelligence official. Hub is believed to be the home of Mr. Mohammed's family.

A week ago, the trail warmed with a tip that he was hiding at a house in the remote Baluchistan capital of Quetta. Mr. Mohammed escaped, but Pakistani intelligence agents learned the name of a Pakistani allegedly working with him -- Ahmed Abdul Qadoos, a 42-year-old member of the country's biggest mainstream religious party, Jamaat-e-Islami. They tracked Mr. Qadoos to a home in Rawalpindi and raided it on Saturday. A relative of Mr. Qadoos's told a local television station that dozens of armed gunmen entered the house around 2:30 a.m. Saturday. The woman said they confiscated a computer and took away Mr. Qadoos.

There were conflicting reports on the exact place of Mr. Mohammed's subsequent arrest. Some reports said he was captured at Mr. Qadoos's house. But a senior Pakistani official said Mr. Qadoos led officials to a hideout on Rawalpindi's outskirts, where Mr. Mohammed was found. U.S. officials said that CIA personnel were present at the raid, although they said Pakistani agents took the lead.

In addition to his alleged role in major al Qaeda operations, Mr. Mohammed is believed by some to have been involved with the murder of Wall Street Journal reporter Daniel Pearl. Mr. Pearl, the Journal's South Asia bureau chief, was kidnapped in January 2002 while reporting a story on Islamic radicalism and later murdered. Two men suspected of keeping Mr. Pearl captive told investigators that three Arab-speakers showed up after the reporter had been held for about a week, and that one killed him, Pakistani police say.

A senior intelligence official said that there is no hard evidence yet that Mr. Mohammed personally ordered or took part in the murder. U.S. authorities say Mr. Pearl's murder will be high on their list of interrogation topics. "We do believe there's a connection," said one U.S. government official.

Mr. Mohammed has been linked to several attacks since Sept. 11, 2001, including the bombing of a Tunisian synagogue in April, which killed 22 people, mainly tourists, and the attack on a French tanker off Yemen in October, which killed one crewman, U.S. officials said. Sometime after Sept. 11 he met with Jose Padilla, a U.S. citizen arrested when he arrived in Chicago last year, to discuss Mr. Padilla's idea for a "dirty bomb" attack in the U.S. using radioactive material. They also discussed attacks on other targets in the U.S., including gas stations and power plants. The CIA learned this from its interrogation of Abu Zubaydah, a senior al Qaeda finance manager who is in custody, a senior U.S. intelligence official said.

Mr. Mohammed's arrest comes at a crucial time for Pakistan's President Musharraf, who is being pressed by conflicting interests ahead of a possible United Nations vote on a second resolution on Iraq's adherence to a U.N. order to disarm. Mr. Musharraf has been vilified at home by political critics and targeted for assassination by Islamic militants who say he has betrayed his country and the Muslim religion. But he also is under pressure from the U.S. to back the international war on terrorism and to support a second U.N. resolution.

The high-profile arrest of Mr. Mohammed could provide evidence of Mr. Musharraf's cooperation with the U.S. even if Pakistan chooses to oppose a U.S. sponsored resolution at the U.N., analysts say.

-- Alan Cullison, Gary Fields and Jess Bravin contributed to this article.

Write to Jay Solomon at jay.solomon@wsj.com⁴, Steve LeVine at steve.levine@wsj.com ⁵and David S. Cloud at david.cloud@wsj.com⁶

Updated March 3, 2003 9:11 a.m. EST