A Kuwaiti lieutenant of Usama bin Laden, Khalid Shaikh Mohammed, was a key organizer of the Sept. 11 terrorist attacks, a senior U.S. counterterrorism official said Tuesday.

"There's lots of links that tie him to 9/11," the official said, speaking on condition of anonymity. "He's the most significant operational player out there right now."

U.S. investigators believe Mohammed planned many aspects of the Sept. 11 attacks, turning bin Laden's calls for dead Americans into reality.

A second U.S. official, also speaking on condition of anonymity, said Mohammed played a key role in planning the attacks as did Abu Zubaydah, the Al Qaeda leader now in U.S. custody. The official acknowledged that Mohammed played a critical role in planning the attacks but said questions remain about the extent of his leadership.

Mohammed was a close associate of Abu Zubaydah, officials said.

Designated one of the FBI's most-wanted terrorists, Mohammed is at large in Afghanistan or nearby, the counterterrorism official said.

Other bin Laden lieutenants are also believed to have helped put together the attacks, the official said. But evidence is mounting that Mohammed was at the center of the operational planning.

Within three months of Sept. 11, according to the official, the FBI learned that Mohammed had moved money that was used to pay for the attacks and since then the United States has gathered other significant evidence pointing to him as the key planner. The official declined to go into detail, citing a need to protect intelligence information.

Mohammed is accused of working with Ramzi Yousef in the first bombing of the World Trade Center, which left six dead in 1993. He and Yousef, hiding in the Philippines, also are accused of plotting in 1995 to bomb several trans-Pacific airliners heading for the United States. Yousef, now serving a life sentence in the United States, also is believed to have planned to crash a plane into CIA headquarters.

The State Department is offering a $25 million reward for information leading to Mohammed's capture. Officials said he continues to plot attacks against U.S. interests.

Mohammed was charged by federal prosecutors in New York in 1996 in connection with the alleged 1995 plot. The FBI describes him as in his mid-thirties, sometimes wearing a beard and glasses, and slightly overweight. His aliases include Ashraf Refaat Nabith Henin, Khalid Abdul Wadood, Salem Ali and Fahd Bin Abdallah Bin Khalid.

U.S. officials have repeatedly said that capturing or killing bin Laden's cadre of lieutenants — men like Mohammed — is a key goal in the war on terrorism. In some ways, they are considered as dangerous as bin Laden: Where Al Qaeda's leader serves as an inspiration to his followers, his top aides conduct the nuts-and-bolts planning of attacks.

The lieutenants are said to pick targets and attack dates, provide money and training to the foot soldiers and overseas cells chosen to carry them out — sometimes at the cost of their own lives — and maintain operational secrecy.

Most of the 19 suicide hijackers are thought not to have known the entirety of the Sept. 11 plot — or that they were going to die — but Mohammed apparently did, the counterterrorism official said.

Mohammed has not been charged in connection with the attacks, which crashed hijacked airliners into the World Trade Center and Pentagon, leaving more than 3,000 dead.

Abu Zubaydah, captured in Pakistan in March, is said to have told U.S. interrogators that the hijacked plane that crashed in Pennsylvania was destined for the White House, suggesting he knew of the planning.

Some of the hijackers trained at Abu Zubaydah's Khalden camp in Afghanistan, the counterterrorism official said. Generally, though, the hijackers trained in groups of one or two at several camps, and they were kept apart from most other trainees.

According to investigators, the Sept. 11 attacks were largely paid for by Shaikh Saiid al-Sharif, also known as Mustafa Ahmed al-Hisawi, who is bin Laden's financial chief. Officials traced a number of financial transactions between him and several of hijackers, but Shaikh Saiid was not believed to have the wherewithal to plan an operation of Sept. 11's magnitude. He is at large.

A fourth bin Laden lieutenant, Tawfiq Attash Khallad, is also suspected of playing a planning role. He met with future hijackers Khalid Almihdhar and Nawaf Alhazmi in Kuala Lumpur, Malaysia, in January 2000, just before Almihdhar and Alhazmi entered the United States. Khallad paid for some of the pair's travel before Sept. 11, the counterterrorism official said.

Khallad, also believed to be a chief planner of the October 2000 USS Cole bombing, remains at large, the official said.

Some key connections have yet to be worked out, the official acknowledged, such as who selected chief hijacker Mohammed Atta and the rest for the operation.

Bin Laden and his top two deputies, Ayman al-Zawahri and Mohammed Atef, were believed to have known about the attacks in advance, by virtue of their rank in Al Qaeda. Al-Zawahri's family was killed by a U.S. airstrike in Afghanistan. It is not known where he is.

Atef, killed by military and CIA airstrikes in November, also had a martyrdom video of one of the so-called "20th hijackers" — the ones who never made it on a plane — at his house. Ramzi Binalshibh was a member Atta's cell in Germany, but was unable to enter the United States. He remains at large.

The official said the video remains one of the key connections between Atta and bin Laden's inner circle.

Bin Laden himself — assuming he wasn't just boasting — admitted foreknowledge of the Sept. 11 attacks on a Nov. 9 videotape of him having dinner with a Saudi sheik, al-Zawahri and some other supporters.

The Associated Press contributed to this report.