Washington ? U.S. counterterrorism officials have identified two key lieutenants of Osama bin Laden including an alleged mastermind of the Sept. 11 attacks as the most active plotters of several al-Qaida attacks during the past year.

While many top al-Qaida 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-Qaida’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-Qaida plots, U.S. officials said 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 a 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-Qaida, is also suspected of ties to the plot.

The Tunisia attack marked al-Qaida’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-Qaida 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 believe 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.

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

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.

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-Qaida’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.