The likely mastermind of the September 11 attacks is a Kuwaiti-born lieutenant of Osama bin Laden who had previously plotted to attack the World Trade Center and bomb several airliners at once, a senior US counterterrorism official says. Khalid Shaikh Mohammed, one of the FBI's most wanted terrorists, is at large in or near Afghanistan, a law enforcement official told The Associated Press on Tuesday. US investigators believe that Mohammed, working under Bin Laden's leadership, planned many aspects of the attack. “There are many links tying him to the September 11 attacks,” the official said, speaking on condition of anonymity. “He was closely involved.” Mohammed is accused of collaborating with Ramzi Yousef in the first bombing of the World Trade Center, which left six dead in 1993. He and Yousef were also accused of plotting in 1995 to bomb several trans-Pacific locations. airliners bound for the United States. Yousef, now serving a life sentence in the United States after being convicted of the 1993 World Trade Center bombing, is also believed to have planned to crash a plane into CIA headquarters. Mohammed was charged by federal prosecutors in New York in 1996 in connection with the alleged 1995 plot. The State Department is offering a reward of up to $25 million for information leading to his capture. Other Bin Laden lieutenants are also believed to have helped organize the Sept. 11 attacks, the official said. But evidence is mounting that Mohammed was at the center of the operational planning. A second American official, also on condition of anonymity, acknowledged that Mohammed played a key role in planning the attacks, but said questions remained 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 high-level organizational roles. Mohammed, 36, is one of the highest-ranking al-Qaeda leaders still at large, officials said, and continues to plan attacks against U.S. interests. Although he was born in Kuwait, local officials say he is a Pakistani citizen and note that people born in Kuwait are not automatically entitled to citizenship. According to the counterterrorism official, within three months of 9/11, the FBI learned that Mohammed had made some financial transactions to finance the attacks; The United States has since gathered significant other evidence pointing to him as the key planner. The official declined to go into details, citing the need to protect intelligence information.