Baghdad, Iraq ? A suicide bomber crashed a cement truck laden with explosives through a side gate at the United Nations compound Tuesday in Baghdad, killing at least 20 people, including Sergio Vieira de Mello, the United Nations’ special representative for Iraq, and wounding more than 100.

Eyewitnesses and officials said the bomber smashed the truck into the corner of the building where de Mello’s second-floor office was. The explosion left a crater about 5 feet deep and 15 feet wide, and an entire corner collapsed. U.N. staff said the powerful blast sent shock waves and debris flying through the structure. Pieces of burned and twisted metal lay on a road more than 200 yards away.

L. Paul Bremer, the U.S. civil administrator for Iraq, arrived on the scene two hours later, visibly shaken, and with tears in his eyes embraced one of de Mello’s aides. Bremer said he thought de Mello was the target of the attack. Bremer declined to say who he thought might have carried out the bombing, but he suggested it was the work of professional terrorists.

“This is an awful time,” Bremer said. “We will leave no stone unturned to find the people who did this.”

Militants suspected

The brazen strike on the top symbol of international cooperation in Iraq renewed fears that despite the presence of nearly 150,000 U.S. and other coalition troops in a country the size of California, no target is safe from terrorist attack. The devastating blast came just 12 days after a car bomb killed 19 people at the Jordanian embassy in Baghdad, and it’s believed to be the largest attack ever against a U.N. facility.

Combined with that bombing and strikes against an oil pipeline in northern Iraq and a water main in Baghdad over the weekend, the U.N. bombing also prompted fears that anti-American militants in Iraq, especially foreign terrorists, are shifting their strategy to include attacks on innocent civilians and vulnerable nonmilitary targets. By striking at the center of international humanitarian efforts in Iraq, the terrorists’ goal appears to be to further destabilize the country and undermine the faith of average Iraqis in the ability of U.S. troops and their allies to protect them.

“The terrorists who struck today have again shown their contempt for the innocent,” President Bush said from his ranch in Crawford, Texas. “They showed their fear of progress and their hatred of peace.”

Bremer said he spoke to U.N. Secretary General Kofi Annan, and Annan assured him that the United Nations wouldn’t be deterred in its mission in Iraq.

Rescue workers and soldiers work outside Baghdad's Canal Hotel, the headquarters for United Nations workers in Iraq. A suicide attacker set off a truck bomb Tuesday outside the hotel, leaving at least 20 U.N. workers and Iraqis dead, including Sergio Vieira de Mello, the chief U.N. official in Iraq.

The United Nations’ role in Iraq has been mainly humanitarian. The U.N. Security Council never ratified the American-led war against Iraq, and the United Nations isn’t playing a governing role in the U.S.-led occupation. The building was guarded, but it wasn’t as well protected as the facilities of the American-led coalition.

‘Evil expression’

While Bremer was at the scene, part of the building collapsed, prompting shouts for him and other officials to move out of danger. He arrived with three members of Iraq’s Governing Council. Council member Akila al Hashimi called the act an “evil expression” of terrorists.

“We are shocked, but not surprised,” Hashimi said. “All Iraqi people, all people of this land of Iraq, have been subjected to this attack. It’s a terrorist attack not only against Iraqis and the people of the U.N., but also people of the whole world.”

De Mello, 55, lauded as a courageous and committed diplomat for his work in postwar Kosovo and East Timor, initially survived the blast, said Saleem Lone, de Mello’s spokesman. Lone said rescuers had found a “gravely injured” de Mello in the rubble and had given him water.

Lone, whose shirt and pants were covered in blood and whose arms were serrated with lacerations, was talking to reporters when informed of de Mello’s death.

He called de Mello, who was scheduled to leave Iraq next month, “a wonderful man” whose “principal goal was to bring an end to the (U.S.-led) occupation” of Iraq and to move the United Nations to the forefront of efforts to rebuild the brutalized country.

Order disobeyed

Four eyewitnesses said they saw a large, yellow cement truck crash through the gates of the compound and explode. The driver appeared to have died in the blast, they said.

A U.S. soldier consoles a woman who was at the blast site at the United Nations headquarters at the Canal Hotel in Baghdad, Iraq, after a suicide attacker set off a truck bomb outside the hotel, according to U.S. officials. Tuesday's bombing killed at least 20.

“The guards asked the truck to stop, but he didn’t obey the order and he forced his way inside the compound. Then everything blew up,” said Saad Ziad, a U.N. worker who was deafened by the blast.

Officials said about 300 people worked in the building, the former Canal Hotel, and the blast took place as many were leaving for the day. Windows also blew out and parts of the ceiling collapsed at the National Spinal Cord Injury Center next door, piling rubble onto dozens of paralyzed patients, said witnesses who pulled them from the wreckage.

More than a dozen Red Crescent ambulances swarmed onto the hospital’s grounds, and more than 40 patients were evacuated, said Mohammed Kassen, a hospital worker who helped with the rescue efforts.

Smoke poured out of the devastated U.N. compound next door as the United Nations’ distinctive blue flag fluttered in the breeze above the rubble. U.S. medical helicopters flew steadily back and forth from a landing zone next to the spinal center, ferrying the dead and wounded to an American medical facility in eastern Baghdad.

Search continues

Scores of Iraqis gathered at the scene, many desperate to know the fate of loved ones inside.

“Please,” a middle-aged woman begged a U.S. soldier, who stood guard on a pedestrian crosswalk over a highway outside the compound. “My daughter is in there. I must see her.”

As darkness fell, American soldiers, coalition security personnel, U.N. staff and Iraqi police swarmed over the scene under bright lights, as four U.S. servicemen, three of them stripped down to their T-shirts, dug through the rubble near where de Mello’s office had been. FBI agents, already in Iraq to investigate the bombing of the Jordanian embassy two weeks ago, are taking a role in the inquiry, according to officials in Washington, who spoke on condition of anonymity.

A group of soldiers with powerful flashlights searched for survivors on the upper floor of the three-story building. Blasted office furniture and equipment, smashed bricks and iron gratings lay scattered for hundreds of feet around. U.N. staff and U.S. soldiers said it was unclear how many people still lay buried under the rubble.