U.S. authorities, still buoyed by the capture of al Qaeda operations chief Khalid Sheik Mohammed, said yesterday they are concerned that his nephews -- the brothers of imprisoned terrorist Ramzi Yousef -- may be positioned to take over planning of future terror attacks.

As FBI agents began running down leads obtained in a search of the house where Mohammed was captured in a suburb of the Pakistani capital of Islamabad, counterterrorism officials said they believe that two of Yousef's brothers, along with a younger cousin, are immersed in planning al Qaeda operations in the United States and Europe.

Even before Mohammed's arrest Saturday, sources said, there were concerns in the U.S. intelligence community that the men had the experience and connections to succeed him. The brothers have been in Pakistan and move easily around the Middle East, officials said, but their whereabouts are unknown.

Mohammed, the self-declared mastermind of the Sept. 11, 2001, attacks and a key conduit for al Qaeda plans, money and personnel in Asia, Europe and the United States, was captured in Rawalpindi Saturday -- perhaps the biggest blow to date against the terrorist network.

U.S. officials said yesterday that his family members have already been involved in some of the same efforts, though they are not believed to have a similar direct relationship with al Qaeda's leader, Osama bin Laden. Mohammed was the link between bin Laden and Ayman Zawahiri, who are in hiding, and the rest of the surviving organization, senior administration officials said.

Yesterday, FBI officials said agents in the United States and abroad were running down lists of names and other leads found with Mohammed. Authorities said they hope records and computer files will lead them to al Qaeda operatives who may be planning new attacks.

Homeland Security Secretary Tom Ridge said yesterday Mohammed was associated with a "significant terrorist plot" aimed at the United States when authorities raised the national threat index last month. That threat has subsided, he said, one reason authorities lowered the alert level a notch last week.

Authorities in Pakistan say they were led to Mohammed by information developed from other arrests, including the apprehension five months ago of Ramzi Binalshibh, who helped coordinate the Sept. 11 attacks from Germany. U.S. officials said yesterday they were also aided by the capture last month in Quetta, Pakistan, of Mohammed Omar Abdel-Rahman, a son of the blind Egyptian cleric currently imprisoned for plotting to blow up New York landmarks. The sheik, Omar Abdel Rahman, inspired the 1993 truck bombing of the World Trade Center, for which Yousef was convicted.

The families of Mohammed and Yousef are from the Baluchistan border area of Pakistan. The men, both nearly 40, grew up in Kuwait. Two of Yousef's brothers, also described as being in their forties, have worked closely with Mohammed in handling al Qaeda communications, travel and financial transfers, sources said.

They are identified as Abd al-Mun'im Yousef and Abd al Karim Yousef. Abd al Karim Yousef, who speaks English, attended North Carolina A&T University with Mohammed during the 1980s. A cousin, Ali Abd al-Aziz, whose age was put at about 25, is also part of the Mohammed clan's network, sources said.

Mohammed's role in al Qaeda became more prominent after its military operations officer, Mohammed Atef, was killed in the U.S. bombing campaign in Afghanistan in November 2001. Mohammed remained in contact with bin Laden over the past year and probably in past months, some analysts said.

Mohammed is being interrogated by the CIA in an undisclosed location outside Pakistan.

"The starting assumption is that he will try to cause as much damage as he can because of his hate for the U.S.," a senior analyst said, by "withholding and distorting information and releasing information that would achieve his goal."

Abu Zubaida, the al Qaeda operative who was wounded when captured in Pakistan a year ago, used his initial questioning to provide a series of false leads, including one that caused an alert for a possible attack on the U.S. banking system.

Mohammed also allegedly supplied funds and personnel to a terror network in Southeast Asia for a 2001 plot, foiled by authorities, to bomb the embassies in Singapore of the United States, Israel and Britain, according to sources familiar with intelligence gathered by U.S. and Canadian authorities.

The sources said that in July 2001, bin Laden met Mohammed Mansour Jabarah, a young Canadian man of Kuwaiti descent who traveled to Afghanistan seeking a jihad mission. Bin Laden saw Jabarah as valuable because he spoke English and held a Canadian passport. He told him to travel to Karachi, Pakistan, to meet with Mohammed, who would provide instructions and funds for an operation.

In August 2001, Jabarah spent two weeks in Karachi with Mohammed, who told him he would act as an al Qaeda conduit for money to finance suicide operations in the Philippines. Jabarah, the sources said, was to act as a liaison between Mohammed and local South Asian operatives. Mohammed gave Jabarah $10,000 for expenses before sending him to meet his South Asian contacts, warning him to "make sure you leave before Tuesday" -- Sept. 11, 2001.

In September, Jabarah met with Riduan Isamuddin, known as "Hambali," leader of al Qaeda in South Asia and operations chief of the militant Islamic group Jemaah Islamiah. Hambali told Jabarah of attacks planned against U.S. and Israeli embassies in the Philippines. Later, the plotters decided those embassies were too difficult to strike and decided to attack the embassies in Singapore. Authorities arrested some of the plotters in December 2001.

In early 2002, Jabarah called Mohammed in Karachi but spoke instead to another al Qaeda operative, Tawfiq bin Attash, a key planner of the October 2000 bombing of the USS Cole. Attash asked Jabarah to help move about 15 al Qaeda fighters returning home to Yemen from Afghanistan. The fighters traveled through Iran, then sailed to Oman, the intelligence sources have been told, where Jabarah was to help smuggle them back into Yemen. Instead, Jabarah was apprehended last March by authorities in Oman and is in U.S. custody.

Correspondent Ellen Nakashima in Jakarta and staff writer Christopher Lee in Washington contributed to this report.