Khalid Sheikh Mohammed was born in Kuwait in either 1964 or 1965, and is one of at least five siblings. He is said to be fluent in Arabic, English, Urdu and Baluchi.
Although born in Kuwait, he is a Pakistani national whose family is from Baluchistan, a region that straddles Pakistan's borders with Iran and Afghanistan.
He was raised in Fahaheel, a busy oil settlement south of Kuwait City, on the road to Saudi Arabia.
According to local press reports, both of his parents had religious callings.
His father, Shaikh Mohammed Ali Doustin Baluchi, who is said to have died decades ago, has been described as a former imam at a mosque.
Mohammed's mother, Halema, was said to have worked cleaning women's bodies for burial. While low paying, this is considered a prestigious job in Islam.
Sheikh Mohammed attended high school at an all-boys school. Teachers and alumni have said they remember him as a studious youth who concentrated on science.
Sheikh Mohammed moved to the United States in 1983, apparently to study engineering. He attended Chowan College, a small Baptist school in North Carolina, before transferring to the North Carolina Agricultural and Technical State University. He graduated in 1986 with a degree in mechanical engineering.
He is also the uncle of Ramzi Yousef, who was convicted in 1997 of bombing the World Trade Center in 1993.
Before his capture, Sheikh Mohammed communicated with Al Qaeda cells around the world by courier, e-mail, coded telephone conversations and shortwave radio. German intelligence even said that when he was forced to hide in rural areas he sent messages by donkey. He's reported to have used more than three dozen aliases.
In September 2002, Pakistan's Inter-Services Intelligence agency brought his two sons, aged 7 and 9, into custody.
Sheikh Mohammed is also believed to be a close associate of Abu Zubaydah, a member of Bin Laden's inner circle and one of Al Qaeda's most senior officials.
He was indicted on terrorism charges in the U.S. District Court for the Southern District of New York in January 1996 and was then placed on the October 10, 2001 intial list of the FBI's twenty-two Most Wanted Terrorists.
The U.S. Government offered a $25 million reward for information leading to his whereabouts. He was caught in Pakistan in 2003, and was eventually brought into U.S. custody.
He was initially kept in a secret prison, but in September 2006 the U.S. government announced it had moved Sheikh Mohammed to Guantanamo Bay.
"He is the kingpin of Al Qaeda," an aide to the Pakistani president said after his capture. "It is a big achievement."
(Profile: Al-Qaeda 'kingpin', BBC, http://www.bbc.co.uk; http://news.bbc.co.uk/2/hi/south_asia/2811855.stm)
(The Plots and Designs of Al Qaeda's Engineer Khalid Shaikh Mohammed, Los Angeles Times)