While revealing few details about the operation that snared Hasan, Brig. Gen. Vincent Brooks, a Central Command spokesman, said the operation took place at night in Baghdad and resulted in no casualties. Hasan was the only person captured, Brooks said.
"The capture demonstrates the coalition's commitment to relentlessly pursuing the leaders of a scattered regime," he added.
It was the second reported arrest of a person on the list of 55 wanted Iraqis and another sign of gradual success in tracking down individuals allegedly involved in Saddam's regime or terrorism.
The first reported captured was Watban Ibrahim Hasan, one of Saddam's three half brothers, who once served as Iraq's interior minister.
Another person on the list of 55, Iraqi science adviser and special weapons chief Lt. Gen. Amer al-Saadi, surrendered over the weekend.
Ali Al Hasan Majid, known as "Chemical Ali" for his role in Iraq's alleged use of chemical weapons against Kurds in 1988, is believed to have been killed in an airstrike.
War commander Gen. Tommy Franks said Sunday that the United States was holding several high-ranking Iraqi prisoners in western Iraq. Neither he nor Pentagon officials would say how many leading Iraqis have been captured.
The biggest uncertainties concern the people at the top of the list: Saddam and his sons Uday and Qusay. It is not clear if they are alive or dead, in Iraq or elsewhere.
Barzan al-Tikriti, like his brother Watban, was seen as a major catch because of the likelihood he could provide information on Saddam's suspected weapons of mass destruction program, one of the major reasons the United States and Great Britain launched the war.
So far, coalition forces have not found any certain evidence that those weapons exist in Iraq.
The third half-brother, Sab'awi Ibrahim Hasan al-Tikriti, reportedly had taken refuge in Damascus, Syria. He is the six of diamonds in the 55-card deck carried by U.S. troops.
The United States has issued stern warnings to Syria not to harbor fugitives from the Saddam regime. Damascus denies any top officials of the former government have been offered sanctuary.
The U.S. also has in custody two important figures not on the list of 55.
One is, Jaffar al-Jaffer, a former nuclear program chief in Iraq, who had fled to an undisclosed Middle Eastern country before turning himself in to U.S. forces. Another is Abu Abbas, wanted for plotting the 1985 Achille Lauro hijacking, who was captured by U.S. forces in Iraq.
Former nuclear weapons inspector David Albright told the Times said Jafar was "always seen as the most important nuclear scientist" in Iraq and "probably the best scientist Iraq has ever produced."
According to the newspaper, Jafar claims to have been tortured and jailed in the 1980s, and then to have agreed upon release to work in developing the bomb. According to the Bush administration, Iraq was very close to developing a nuclear weapon at the time of the 1991 Gulf war.
The White House claims Iraq has tried to reconstitute its nuclear program since 1991, but United Nations inspectors found no evidence of this. One key piece of evidence cited by the United States — a document indicating that Iraq tried to buy uranium from Niger — is suspected of being a forgery.
The elegant, British-educated al-Saadi had been wanted because he was a special weapons adviser to Saddam and oversaw Iraq's chemical program in the past. He is believed to have in-depth knowledge of other weapons programs as well.
Al-Saadi told the German television station ZDF he had spent the war in his cellar and emerged after he saw a British TV report that he was being sought.
He said he had no information on what happened to Saddam and repeated his assertion, made often in news conferences before the U.S.-led war started on March 20, that Iraq was free of weapons of mass destruction.
This week, American troops also raided the home of Dr. Rihab Taha, nicknamed "Dr. Germ," a microbiologist suspected of running Iraq's biological weapons program. Her whereabouts are unknown. She is not among the 55 wanted leaders.