Washington ? Investigators believe they have identified a Kuwaiti lieutenant of Osama bin Laden as the likely mastermind of the Sept. 11 terrorist attacks, a senior U.S. counterterrorism official said Tuesday.

Khalid Shaikh Mohammed, designated one of the FBI’s most-wanted terrorists, is at large in Afghanistan or nearby, the law enforcement official told The Associated Press.

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

“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.”

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.

A second U.S. official, also speaking on the condition of anonymity, acknowledged that Mohammed played a critical role in planning the attacks but said questions remain about the extent of his leadership. The official said other bin Laden lieutenants, including Abu Zubaydah, now in U.S. custody, are also believed to have played top organizational roles.

According to the counterterrorism official, within three months of Sept. 11, 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-30s, 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.