A huge explosion that may have been the work of a homicide bomber ripped through U.N. headquarters in Baghdad Tuesday, killing 20 people and injuring scores of others.
The U.N. Security Council (search) called the incident a "terrorist attack."
Among those who died was U.N. Iraq representative Sergio Vieira de Mello (search), one of the highest-ranking officials at the United Nations.
At least one American was also killed in the blast: Richard Hooper, 40, of Walnut Creek, Calif., the special assistant to the U.N. Undersecretary-General for Political Affairs. A State Department consular officer said the U.S. military was working to identify any other Americans who may have been injured in the explosion or who were in its vicinity.
Other U.N. workers who were killed came from the Philippines, Egypt, Britain and Canada.
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At least 100 others were wounded, according to the most recent reports. Survivors reported other victims still buried.
"Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq for one purpose only: to help the Iraqi people recover their independence and sovereignty, and to rebuild their country as fast as possible, under leaders of their own choosing," U.N. Secretary-General Kofi Annan (search) said in a statement.
A bomb in a large truck -- possibly a cement mixer -- caused the blast around 4:30 p.m. local time.
"The explosion was caused by a massive truck bomb," said Bernard Kerik (search), the senior U.S. law enforcement official in Baghdad. "We have evidence to suggest it could have been a suicide attack."
Asked if Al Qaeda was behind the attack, Kerik said, "It's much too early to say that. We don't have that kind of evidence yet."
The blast may have specifically targeted Vieira de Mello, said L. Paul Bremer, who heads the U.S.-led administration in Iraq. "The truck was parked in such a place here in front of the building that it had to affect his office," Bremer said.
The blast occurred at the Canal Hotel (search), home to the U.N. mission in Iraq. U.N. workers, who lived and worked there, said there were up to 300 workers inside the building at the time of the explosion.
The attack stunned an organization that had long been welcomed by Iraqis, even by many who protested the presence of U.S.-led occupation forces.
Except for a newly built concrete wall, U.N. officials at the headquarters refused the sort of heavy security that the U.S. military has put up around some sensitive civilian sites. The United Nations "did not want a large American presence outside," Salim Lone, the U.N. spokesman in the Iraqi capital, said.
Denis Halliday (search), former U.N. secretary general and former coordinator of humanitarian affairs in Iraq, told Fox News a great majority of people in the building were Iraqi citizens working with the U.N.
"I think this is the worst case I've heard of … I've never heard of a bombing of this nature of a U.N. building," Halliday said.
U.N. officials vowed to continue their mission in Iraq. But the blast, the shock at being targeted and the death of a rising star beloved in the organization struck deep. All the national flags that ring the U.N. headquarters' entrance in New York were removed from their poles, and the blue-and-white U.N. flag was lowered to half staff. Staffers, tears in their eyes, gathered in hallways and watched in shock as televisions reported on his death.
The cement mixer apparently drove through a heavy fence and exploded in or around the lobby, destroying about one-third of the building. Iraqis said the blast blew out windows as far as a mile away.
Bremer walked through the scene of destruction as workers dug through the rubble with their hands. There was a 15-yard-wide hole in the ground.
"We will leave no stone unturned to find the perpetrators of this attack," he said.
Bremer arrived with Lt. Gen. Richardo Sanchez, commander of U.S. forces in the region, and members of the Governing Council, including Adnan Pachachi, (search) who had served as foreign minister in the Iraqi government that was overthrown in the 1968 Baath Party coup.
One entire corner of the hotel was blown away. Emergency workers from a nearby hospital, which also suffered damage, were digging, looking for survivors and victims.
One wounded man had a yard-long, inch-thick aluminum rod driven into his face just below his right eye. He was a security consultant for the International Monetary Fund, saying he had just arrived in the country over the weekend.
"I can't move. I can't feel my legs and arms. Dozens of people I know are still under the ruins," Majid Al-Hamaidi, 43, a driver for the World Bank, cried out.
Vieira de Mello, who was trapped under the rubble for hours before he died, had been talking to rescue workers outside the rubble with his cell phone. The 55-year-old veteran diplomat, serving in what one U.N. spokesman called the world body's toughest assignment, was meeting with other U.N. officials in his office when the explosion brought the room down around them. Workers gave him water as they tried to extricate him. Hours later, the United Nations announced his death.
"We lost contact and eventually it was confirmed to us that he had died," Annan spokesman Fred Eckhard told Fox News. "It's a tragic personal loss but it's also, on a professional and political level, a big loss for the U.N.'s efforts in Iraq … Sergio was one of the big achievers … that's why he was chosen for this job, which is one of the toughest, if not the toughest" job in that country.
Annan's office said the U.N. humanitarian coordinator in Iraq was in the building at the time of the explosion and was slightly injured. Benon Sevan, the undersecretary general, was also in the building but was not hurt. A senior UNICEF official also was seriously wounded.
Defense officials told Fox News that a "handful of U.S. troops were on the other side of the building at the time of the blast." There is a large contingent of U.S. troops around the area.
Most of the security and checkpoints at the building were handled by U.N. personnel and Iraqi Civil Defense members.
The United States dispatched a "Quick Reaction Force" (search) and closed off the area, offering medical assistance and help searching for survivors.
President Bush, who was playing golf in Waco, Texas, cut short his golf game at the 12th hole and returned to his Crawford ranch
In a later address, Bush said the blast reflected the desperation of Saddam Hussein's followers and called the bombers "murderers."
"The terrorists that struck today again showed their contempt for the innocent. They showed their fear of progress and their hatred of peace. They're the enemies of the Iraqi people. They're the enemies of every nation that seeks to help the Iraqi people.
"The civilized world will not be intimidated and these terrorists will not determine the future of Iraq … they are testing our will; it will not be shaken."
Secretary of State Colin Powell telephoned Annan to express sympathy and concern and to volunteer help.
Tuesday's explosion almost mirrored the car bomb that rocked the Jordanian Embassy in Baghdad (search) on Aug. 7, which killed 11 people and left more than 50 wounded.
U.S. forces have been focusing on trying to put down Saddam Hussein loyalists thought to be behind the guerrilla campaign against American troops. But the military has also warned of foreign Islamic militants slipping into the country and has said an Al Qaeda-linked group, Ansar al-Islam was a possible suspect in the Jordanian Embassy bombing.
Dia'a Rashwan, an expert on radical Islam at Egypt's Al-Ahram Center for Political and Strategic Studies, said the attack fits "the ideology of Al Qaeda. They consider the U.N. one of the international actors who helped the Americans to occupy Palestine and, later, Iraq."
U.N. spokesman Fred Eckhard said that if Tuesday's attack was confirmed to be a homicide bombing "it would, to my knowledge, be the first on a U.N. facility." It was the worst attack on a U.N. facility since Israeli forces, responding to a Hezbollah attack, bombarded a U.N. compound at Qana in southern Lebanon in April 1996, killing 91 refugees.
Maj. Gen. Ray Odierno, commander of the Army's 4th Infantry Division north of Baghdad, last month warned his troops to prepare for the possibility of car bombings by supporters of Saddam Hussein and other anti-American forces.
"They are going after softer targets, because they know they're ineffective against military targets," Odierno said July 25.
U.S. administrators and the military "do believe that there are professional terrorists, foreign regime leaders … coming in from the border of Iraq," Sen. Kay Bailey Hutchison, R-Texas, told Fox News from Iraq. "But we cannot cut and run. We must stay here and build the security of this country … the escalation of terrorists attacks are of great concern to everyone."
Fox News' Bret Baier, Steve Harrigan, Ian McCaleb, David Lee Miller, Liza Porteus, Teri Schultz and Dan Springer and The Associated Press contributed to this report.