U.S. counterterrorism officials have identified two key lieutenants of Usama bin Laden -- including an alleged mastermind of the Sept. 11 attacks -- as the most active plotters of several Al Qaeda attacks during the past year.

While many top Al Qaeda leaders went into hiding after Sept. 11, Khalid Shaikh Mohammed and Abd al-Rahim al-Nashiri have taken the lead in arranging new attacks with cells in the field, U.S. officials said, speaking on condition of anonymity.

Mohammed, a Sept. 11 organizer who has risen to be one of bin Laden's top planners, and al-Nashiri, Al Qaeda's Persian Gulf operations chief, are among roughly two dozen key lieutenants being sought by the CIA, FBI and military in a worldwide manhunt.

While last week's capture of Ramzi Binalshibh in Pakistan may shed light on both the Sept. 11 attacks and ongoing Al Qaeda plots, U.S. officials say he was not a leader but an aide to Mohammed. Officials hope that by tracking down the leaders they can disrupt terrorist plots and the multiple cells under their command.

Mohammed, a Kuwait-born Pakistani national, has been linked to the April 11 suicide truck bombing of the Djerba synagogue in Tunisia. At least 19 tourists, mostly Germans, were killed.

The suspected bomber, Nizar Naouar, spoke by phone with Mohammed about three hours before the attack, German officials said. Bin Laden's son Saad, seen as a rising star in Al Qaeda, is also suspected of ties to the plot.

The Tunisia attack marked Al Qaeda's first successful strike since Sept. 11. The suicide bombing of the U.S. consulate in Karachi in June is also believed to be an Al Qaeda operation, but who commanded it has not been determined.

Mohammed, who is on the FBI's most-wanted terrorists list, has been charged in connection with plots in the Philippines to bomb trans-Pacific airliners and crash a plane into CIA headquarters. Those were broken up in 1995. He is believed to related to Ramzi Yousef, who was convicted for his role in the 1993 World Trade Center bombing.

"He's the most significant operational player out there right now," said a senior U.S. counterterrorism official, speaking recently on the condition of anonymity.

U.S. counterterrorism officials believed Mohammed was in the Afghanistan-Pakistan region as recently as June.

The capture of Binalshibh, a Yemeni and planner in the Sept. 11 attacks, probably has set Mohammed on the run, said Vince Cannistraro, a former CIA counterterrorism official.

"Khalid Shaikh Mohammed is probably in touch with all the cells, through other lieutenants," he said. "(Binalshibh) will know where he is -- or at least, where he was."

Al-Nashiri is more of a mystery. A Saudi who is also known as Umar Mohammed al-Harazi and Abu Bilal al-Makki, he is considered a step below Mohammed in Al Qaeda's hierarchy.

He seems to have a particular hatred for the U.S. Navy, and is suspected of links to plots on four naval targets during the last three years.

Most recently, he has been tied to a failed Al Qaeda plot to bomb U.S. and British warships crossing the Strait of Gibraltar, U.S. officials have said. Three Saudis were arrested in Morocco in June in connection with that plot.

He is also suspected of being behind plans to bomb the 5th Fleet Headquarters in Bahrain, a plot revealed in January by a former Al Qaeda training camp commander captured by Pakistan after fleeing Afghanistan.

The 5th Fleet has responsibility for the Persian Gulf and provides ships for the operations of U.S. Central Command, which is running the war effort in Afghanistan. It also supports the enforcement of the no-fly zone over southern Iraq, the U.N. economic embargo against Iraq and the monitoring of sea traffic from the Arabian Sea to the Gulf of Oman and the Persian Gulf.

The fleet headquarters went high alert this week in response to new threats tied to the Sept. 11 anniversary. It's unclear whether al-Nashiri is linked to the alert.

Al-Nashiri is believed to be a mastermind of the October 2000 bombing of the destroyer USS Cole, which was hit by a small boat full of explosives at the port in Aden, Yemen.

He is similarly thought to be behind the attempt to bomb the USS The Sullivans nine months earlier at Aden, which failed when the suicide boat, overloaded with explosives, sank. U.S. counterterrorism officials also suspect he is tied to the 1998 bombings of two U.S. embassies in East Africa.

Officials have said al-Nashiri gave telephone orders to the bombers from the United Arab Emirates.

U.S. officials believe he was in Ghazni, Afghanistan, around the time the war began last October. He is thought to have moved to Pakistan when the Taliban fell.

A third top bin Laden lieutenant, Abu Zubaydah, remained active in plotting terrorist attacks after Sept. 11, but he was captured in March in a joint U.S.-Pakistani raid in Faisalabad, Pakistan.

One of Zubaydah's associates, Omar al-Farouq, was Al Qaeda's chief of operations in Southeast Asia before he was captured and turned over to U.S. authorities. His warnings led in part to the Sept. 10 worldwide terrorism alert.