U.S. forces on Thursday said they believe they have found the main Fallujah headquarters of the terrorist group headed by Al Qaeda-linked Jordanian terrorist Abu Musab al-Zarqawi (search).

Meanwhile, a group attacked U.S. Marines and Iraqi government forces from a house inside the terror hotbed city on Thursday, killing one Marine and one Iraqi soldier, a U.S. military official said. One Marine and one Iraqi soldier also were wounded.

U.S. and Iraqi forces also arrested 104 suspected guerrillas in an insurgent neighborhood in central Baghdad, including nine who are believed to have fled Fallujah, Interior Ministry spokesman Sabah Khadim said. Most were Iraqis, although Syrians and non-Iraqi Arabs were among the group, he said.

And four people were killed when guerrillas detonated a car bomb near a U.S. military convoy in Baghdad and a roadside bomb exploded at a job-recruiting center in Kirkuk.

As for the supposed Zarqawi headquarters, soldiers walked through an imposing building with concrete columns and with a large sign in Arabic on the wall reading "Al Qaida Organization" and "There is no God but Allah and Muhammad is his messenger." The video footage was shot by an embedded CNN cameraman.

Inside the building, U.S. soldiers found documents, old computers, notebooks, photographs and copies of the Quran.

Al-Zarqawi last month renamed his group Al Qaeda in Iraq, and his followers have been blamed for a number of deadly bombings and beheadings of foreign hostages, including three Americans and a Briton. The United States has offered a $25 million reward for his capture or killing - the same amount as for Usama bin Laden.

In neighboring Jordan, authorities detained al-Zarqawi's nephew near the border with Iraq, a distant relative and a clergyman close to the family said Thursday.

Also, the senior U.S. Marine commander in Iraq said the U.S.-led offensive launched last week in Fallujah has "broken the back of the insurgency" by seizing their main base of operations.

"We feel right now that we have, as I mentioned, broken the back of the insurgency. We've taken away this safe haven," Lt. Gen. John Sattler told reporters at the Pentagon in a video teleconference from Fallujah.

Sattler cautioned, however, that insurgents remained a threat and that Fallujah is not yet completely in U.S. and Iraqi government control.

"The town is not quite secure at this point," Sattler said. He later changed his assessment, saying, "The town of Fallujah is secure, but we're in the search-and-clear phase that will make it safe — relatively safe is the best word."

Sattler said the total U.S. death toll so far in the Fallujah offensive, which began Nov. 7, stands at 51, with about 425 wounded in action.

Bomb-making materials and improvised explosive devices need to be recovered or disarmed, he said, and the Marines are still looking for remaining insurgents who may be lying low with the intention of disrupting efforts to reconstruct the city.

He said U.S. forces have found a number of documents from the insurgents' command posts inside the city that lists the names of some of their fighters, including some from outside of Iraq.

Elsewhere, insurgents fired mortars at the provincial administration offices in the northern city of Mosul (search), wounding four of the governor's guards, the U.S. military said.

Governor Duraid Kashmoula was unhurt in the attack, said spokesman Lt. Col Paul Hastings. Initial reports said the mortar attack landed near a fuel truck, setting it ablaze, Hastings said.

The rest of Mosul, Iraq's third largest with more than a million residents, remained calm for a second day since the U.S.-led offensive operation began on Tuesday to wrest control of the western part of the city from insurgents.

Meanwhile, condemnation of the apparent killing of kidnapped British aid worker Margaret Hassan continued to be heard. Marines found the mutilated body of what they believe was a Western woman on a street in Fallujah recently but the body has not yet been identified, although it is believed to be that of Hassan.

In western Baghdad, a car bomb exploded near the Yarmouk police station as a U.S. armored vehicle drove by, said police Capt. Ahmed Shihab. Two people were killed and five wounded by the blast, he said. The U.S. military had no immediate information on casualties.

In Kirkuk, two civilians were killed and three injured Thursday when a roadside bomb exploded near a job recruiting center and a bus terminal in the city's center, said Gen. Anwar Mohammed Amin with the Iraqi National Guard. Kirkuk is 180 miles north of Baghdad.

Last week in Mosul, gunmen stormed police stations, bridges and political offices, overwhelming police forces who, in many places, failed to even put up a fight. Some officers also allegedly cooperated with insurgents.

The U.S. military said that up to 2,500 U.S. and Iraqi troops met "little resistance" during operations to re-secure police stations and key bridges in Mosul from the insurgents.

Thursday's attacks came a day after a wave of violence in Iraq's Sunni Muslim (search) heartland killed at least 27 people and U.S. forces pursued the remaining holdouts in the former insurgent bastion of Fallujah.

While U.S. and Iraqi forces have retaken insurgent strongholds in Fallujah and Mosul, violence continues to erupt in Sunni Muslim-dominated areas of Iraq.

Gen. John Abizaid, the commander of all U.S. forces in the Middle East, said during a visit to Iraq this week that the Fallujah offensive was a major blow to the insurgents, and he said the only way the U.S. forces and their Iraqi allies can be defeated is if they lose their will.

"But we are also under no illusions. We know that the enemy will continue to fight," he said.

Hassan's Supposed Death Condemned

Australian Prime Minister John Howard said Thursday a body found in the strife-torn Iraqi city of Fallujah was likely that of Hassan - repeating the belief of British officials and Hassan's family.

"The body found in Fallujah appears to have been Margaret's and the video of the execution of a Western woman appears on all the available information to have been genuine," Howard told Parliament but did not say which body he was referring.

Hassan, 59, director of CARE International's operations in Iraq, was a British citizen born in Ireland and married to an Iraqi man.

The body, clothed in what appeared to be a purple, velour dress, was wrapped in a blanket, with a blood-soaked black cloth nearby. As of Thursday, the U.S. command said the body had not been identified.

When questioned outside Parliament about his comments, Howard did not confirm which body he was talking about and said no remains had been yet returned to authorities.

"This latest example of cruelty and brutality reminds us that there can be only one answer to terrorism and that is the completely uncompromising and unconditional one," he said.

The Arab satellite station Al-Jazeera said early this week it had received a videotape showing the murder of a woman believed to be Hassan, a British-Irish national who had lived in Iraq for three decades.

The White House released a statement voicing sympathy for Hassan's family.

"We strongly condemn the abduction and murder of this prominent humanitarian," the statement read.

"Mrs. Hassan worked tirelessly and with great compassion for more than 25 years to assist the poor and disadvantaged in Iraq, particularly children. As a humanitarian she directed programs to serve the neediest through independent, non-governmental organizations and was not involved in the political process or military activity in Iraq. Her death is a great loss to the Iraqi people and the world."

The Associated Press contributed to this report.