BAGHDAD -- A suicide bomber rammed a cement truck loaded with explosives into the U.N. headquarters here Tuesday, toppling a wing of the building in a thunderous, dust-choked roar and killing the top envoy and at least 19 other people. Scores more -- foreigners and Iraqis -- were injured.

Iraqi policemen and firefighters and American soldiers dug with their bare hands through mounds of cracked concrete in a frantic search for trapped victims.

Among those missing for hours and found dead under the debris was Sergio Vieira de Mello, the top U.N. envoy in Iraq. U.S. officials on the scene said the Brazilian diplomat may have been the intended target because the bomber struck so close to his office.

It was the first suicide bombing in Iraq since the U.S. takeover -- and the second major attack against a civilian target in Iraq this month. Violent opposition to foreign occupiers is posing an increasingly deadly, unpredictable threat to U.S. efforts to stabilize and rebuild the country.

By targeting the United Nations, a world body that did not participate in the U.S.-led war to oust Saddam Hussein, the perpetrators undermined any sense of security here and significantly broadened their guerrilla war.

The U.N. compound was a "soft" target, more vulnerable than any of the U.S. installations here. Housed in what had been the Canal Hotel, the headquarters had no protective berms and no massive U.S. military presence. In fact, many U.N. officials had been keen not to be too closely associated with the occupation.

No group initially claimed responsibility for the attack, which follows a string of assaults on oil pipelines, a water main and the Jordanian Embassy -- as well as daily hit-and-run ambushes of U.S. troops.

President Bush, speaking from his Texas ranch, branded the attackers "enemies of the civilized world" and warned that they will not "determine the future of Iraq."

L. Paul Bremer III, the U.S. civilian administrator in Iraq, rushed to the scene of the bombing, tears in his eyes. He later condemned "the ugly face of terrorism" and said: "The people who did this attacked not the Americans, not even the coalition, not even just the Iraqis, but innocent men and women who were simply trying to make life better for the Iraqis."

Bremer, who has said Iraqi insurgents run the gamut from members of the deposed Hussein regime to foreign mercenaries and possibly elements of the al-Qaida terrorist network, vowed to "leave no stone unturned" to find and punish those guilty of the attack.

There was speculation that the U.N. may have been attacked because Vieira de Mello had recently expressed support for U.S. policy and the new Iraqi Governing Council.

The United Nations identified seven of the dead -- all foreigners, including Richard Hooper of Walnut Creek, Calif.

Blood streaming from his right shoulder, Fawzi Hamdani managed to escape from the U.N. compound where he had been waiting in the parking lot to pick up a friend. About 300 people were working inside the converted hotel when the bomber attacked, shortly before 4:30 p.m., quitting time.

The force of the explosion collapsed an entire corner of the three-story U.N. building, its thick concrete wall flapping over like a piece of burlap, and knocked the ceiling down in a next-door hospital for patients with spinal cord injuries. Windows were blown out of homes for blocks.

Hamdani said he saw the truck that he believes carried the explosives to their mark. The driver, he said, was a young, clean-shaven man in a white T-shirt who did not emerge from the vehicle. "He was determined for this task," Hamdani said.

The truck, described by several witnesses as a cement mixer, apparently crashed through the gate of a cinder-block wall being built to shield the compound and then plowed into a side of the building, directly below Vieira de Mello's third-floor office.

U.S. officials described the attack as a suicide bombing that, given the size of the explosive and its devastating impact, required a level of sophistication and planning not necessarily seen in assaults before now. The U.S. military said the bomb contained 500 pounds of powerful C4 explosives.

A U.S. intelligence official said investigators had yet to identify who was responsible for detonating Tuesday's car bomb and the one at the Jordanian Embassy 12 days earlier.

But the official, who spoke on condition of anonymity, said investigators were focusing primarily on two groups: former Baath Party regime members and a local al-Qaida affiliate known as Ansar al Islam.