American authorities say Khalid Shaikh Mohammed masterminded behind the Sept. 11, 2001, attacks for Osama bin Laden, and is now considered bin Laden's second in command.
Mohammed, a Kuwait-born Pakistani national on the FBI's most-wanted terrorists list, has been linked to the April 11, 2002 suicide truck bombing of the Djerba synagogue in Tunisia where at least 19 tourists, mostly Germans, were killed.
He has been charged in connection with plots in the Philippines to bomb trans-Pacific airliners and crash a plane into CIA headquarters. Those were broken up in 1995. He is believed to be related to Ramzi Yousef, who was convicted for his role in the 1993 World Trade Center bombing.
"I would say that he's extremely dangerous," said Jack Cloonan, a New York-based senior terrorism investigator for the FBI, who retired earlier this year and is now an ABCNEWS consultant. "He's the person who is going to attack the United States."
In 1996, the FBI tracked Mohammed, under indictment on charges of terrorism, to Qatar's capital city, Doha, and was within hours of capturing him.
Cloonan, who was the lead FBI agent on the case, said a specially equipped government executive jet, complete with blacked-out windows, was standing by to transport Mohammed.
"We had located Khalid Sheikh," Cloonan told ABCNEWS. "We were prepared to fly the plane in and to take him out."
But Cloonan says Mohammed was tipped off shortly after Qatar officials were told of the plan and headed for the airport.
"Somebody had leaked the information to Khalid Sheikh and he left," said Cloonan. "It certainly, in my judgment, [was] a significant opportunity that was missed."
Qatari Royal Family Leak
The American military is already a major presence in Qatar and says the royal family has cooperated fully. If there is a war with Iraq, Qatar is where U.S. Central Command will be located.
But at least one member of the royal family is known to have regularly provided safe haven for numerous al Qaeda operatives, including bin Laden.
"Certainly it is well-documented that there is one member of the Qatar royal family that supports al Qaeda," said Rohan Gunaratna, the author of the book Inside Al Qaeda and professor at St. Andrews University in Scotland.
According to American intelligence officials, Abdallah bin Khalid al-Thani, a member of the Qatari royal family tipped off Mohammed. He is listed on government Web sites as the current minister of the Interior for Qatar and is known as a radical Islamic fundamentalist with ties to al Qaeda.
Mohammed is believed to have fled Qatar with a passport provided by that country's government. He is also believed to have been given a home in Qatar as well as a job at the Department of Public Water Works. Officials also said bin Laden himself visited Abdallah bin Khalid al-Thani in Qatar between the years of 1996 and 2000.
One former CIA official who preferred to remain anonymous said the connection went beyond al-Thani and there were others in the Qatari royal family who were sympathetic and provided safe havens for al Qaeda.
"We had an opportunity to pick up a very, very bad man and a very dangerous man," said Cloonan. "He still to this day has an impact on U.S. national security."
It's an extremely sensitive situation for the U.S. military, given the role Qatar would play in a war against Iraq.
The government of Qatar or its interior minister didn't return repeated calls for comment.