Unlike Osama bin Laden, he's not a household name - at least not until now.
But no individual's capture could have wider implications for America's war on terrorism than that of Khalid Sheik Mohammed.
Apprehended in Pakistan this weekend and now facing interrogation in US custody overseas, Mohammed is considered to be Al Qaeda's foremost operational leader, with unrivaled knowledge of the organization's plans and personnel.
The news of Mohammed's capture could help on several fronts, from the manhunt for bin Laden to the efforts to crack "sleeper" cells in the US and elsewhere. For one thing, Al Qaeda members may now feel compelled to change locations, making them more vulnerable to capture.
Moreover, as the Bush administration struggles to build international backing in a controversial showdown with Iraq, Mohammed would also know something that could either bolster or beset that effort: the degree of any Iraqi involvement with Al Qaeda operations.
"This arrest is likely to have profound repercussions on Al Qaeda, and perhaps even on bin Laden and his continued ability to avoid apprehension," says Bruce Hoffman, an expert on terror at the Rand Corp. "Mohammed has been at the vortex of every major operation going back a decade - from the first bombing of the World Trade Center to 9/11 to the most recent incidents."
At the very least, experts say Mohammed's arrest removes a key link in the command structure of the Al Qaeda network, a crucial link that translated Mr. bin Laden's threats into operational details.
Nabbed with two compatriots in a joint FBI-Pakistani sting, he would know virtually every operation in the planning stages - including those in the US.
"Depending on who interrogates him and where, several Al Qaeda operations in the planning, preparation, and execution may be disrupted," says Rohan Gunaratna, an Al Qaeda expert at St. Andrews University in Scotland. "As head of the military committee of Al Qaeda, he knows all the key regional leaders and assets ... in at least 98 countries."
If information is pried from him or from computer disks captured with him, it could fill in details on past and future attacks, their financing, and even how much control the Al Qaeda network exerts on minions worldwide.
The conventional thinking has been that after Al Qaeda was driven from Afghanistan, its leadership went into hiding - unable to communicate and exert control, and that attacks since have been planned and carried out by smaller, disparate local groups.
But Mohammed is the latest of some 400 Al Qaeda members detained in Pakistan, including two other high-level leaders. Abu Zubaydah, Al Qaeda's field operations commander, was captured in a shoot-out in March 2001. Ramzi Binalshibh, who allegedly headed the 9/11 hijackers cell in Germany, was captured in a joint FBI-Pakistani string operation in September 2002.
"Given such a critical mass of Al Qaeda leaders seized in Pakistan, that might reveal a centralized control over operations," Mr. Hoffman says. "They might not have stayed close to Afghanistan only because it kept them from being caught. They might have tried to create a command and control that was pretty effective."
Indeed, Mohammed's arrest puts renewed focus on Pakistan as a key link in the terror war - and on the state of Washington-Karachi cooperation.
While the White House was quick to congratulate Pakistan for its key role in the arrest, the very fact that it occurred in Pakistan underlines a perception which that nation has worked hard to shake: that Pakistan remains a relatively safe haven for Al Qaeda.
It is a point of concern both for US military officials in Afghanistan and for Pakistani liberals who worry about the growing role of Islamist parties in Pakistan, who often voice support for fugitive Taliban leaders.
One senior US military officer in Afghanistan, speaking on condition of anonymity, said that the Pakistani government could clearly be doing more to round up Al Qaeda. "Our bases along the border are being shelled every day from the Pakistani side, so clearly, they could be doing more to stop that," said the officer. "We're grateful for what they're doing, but they could be doing more to keep their country from being turned into a safe haven."
But Pakistani officials say that Mohammed's capture shows that Pakistan is far from a safe haven for terrorists. "It's not a question of how was he in Islamabad or Rawalpindi," says Maj. Gen. Rashid Qureshi, spokesman for Pakistani President Gen. Pervez Musharraf. "It's the fact that he has been identified and captured that is significant."
To be sure, this is a major blow to the Al Qaeda network. While the arrest of one man will not cripple the organization, members will have to presume their locations and operations are compromised, and this will throw them into disarray for some time. In all, intelligence officials estimate that more than one-third of the Al Qaeda leadership has been either detained or killed. "Operationally, his arrest is the most significant since 9/11," Mr. Gunaratna says. "It will impact more on operations than anyone else's arrest, including bin Laden's."
Mohammed was born to Pakistani parents in Kuwait in 1965, studied in the US, and is considered a master language and disguise. He had a $25 million reward on his head.
Khalid Sheik Mohammed, according to intelligence officials and experts, was a principal planner of these Al Qaeda strikes:
• The October 2002 Bali bombing that killed nearly 200 people.
• The April 2002 attack on a historic synagogue in Tunisia that killed 21.
• The January 2002 murder of Wall Street Journal reporter Daniel Pearl.
• The Sept. 11 attacks on the World Trade Center and Pentagon.
• The October 2000 bombing of the USS Cole in Yemen, where 17 US servicemen died.
• The 1998 US Embassy bombings in Tanzania and Kenya.
• The 1995 plot in the Philippines to bomb several airliners simultaneously, and crash an airplane into CIA headquarters.
• The first World Trade Center bombing in 1993. Mohammed's nephew, Ramzi Yousef, has been convicted in connection with that attack.