BAGHDAD -- The giant truck bomb that ripped apart the United Nations headquarters on Tuesday was composed of more than 1,000 pounds of military-grade munitions of the kind found in Saddam Hussein's arsenal, according to FBI investigators combing through the rubble yesterday.

Questions and accusations mounted among stunned UN staff about why they had been left so vulnerable to a devastating attack, which killed up to 20 people and injured more than 100. Underlining their suspicions that there had been major security flaws, a leading Iraqi politician said that extremists had met five days before the blast to plan a big attack in Baghdad, possibly against the UN.

Stricken by their loss, several employees flew to Jordan in a partial evacuation, even as the UN chief vowed that terrorists would not drive the organization from Iraq.

"As I've indicated, we shall not be deterred," said UN Secretary General Kofi Annan to reporters, after flying back to New York from Stockholm for emergency Security Council talks. "We have a responsibility to stabilize Iraq. It's a responsibility for all of us, and we're going to keep at it until we succeed."

The World Bank and International Monetary Fund, however, announced they would withdraw expatriate staff from Iraq. Other UN agencies said their staff members could leave if they wished. About 60 percent of UN employees in Iraq are foreigners.

With unusual candor, Annan admitted to reporters that UN officials might have mistakenly refused security assistance from the US military in Iraq. "If they did, it was not correct, and they should not have been allowed to turn it down," he said.

For the first time, evidence emerged yesterday that the attack had been planned by Saddam Hussein loyalists looking to strike at nonmilitary targets in Baghdad.

Ahmed Chalabi, leader of the Iraqi National Congress, told reporters here yesterday that members of Hussein's crushed regime met with extremists last Thursday to plan a "large-scale act in Baghdad" against a civilian "soft target," possibly including the United Nations. Chalabi's group told US officials about the meeting, he said, but had no specific information about an attack on UN offices.

The chief FBI agent in Iraq, who was brought in two weeks ago to investigate another suicide bombing at the Jordanian Embassy, said the attackers appeared to have access to Hussein's mammoth weapons stores.

"These were weapons that would have been in Iraq's arsenal before the conflict," said Thomas Victor Fuentes, who is leading the UN bomb investigation. "This was no homemade bomb," he told reporters after a daylong hunt through the ruined complex.

The truck's lethal cargo included a 500-pound Soviet-made bomb from the 1970s or 1980s, as well as Soviet-made mortar rockets and artillery shells, Fuentes said. A hand grenade was also found.

The weapons were packed together on a flat-bed truck, contrary to initial witness accounts of a cement truck, which drove down a public lane abutting the UN's offices in the old Canal Hotel in east Baghdad.

"The road was totally unsecured," Fuentes said.

That lack of security continued even after a similar truck bomb obliterated the Jordanian embassy here on Aug. 7, killing 17 people. Since then, the US administrator here, L. Paul Bremer III, has said that foreign terrorists have established a foothold in Iraq since the war's end.

On Tuesday, the truck carrying the lethal load stopped next to a stone wall enclosing the UN compound, directly under the third-floor office of the UN's chief diplomat in Iraq, Sergio Vieira de Mello. The suicide bomber then detonated the explosives, sending glass and twisted metal hurtling across a half mile and blowing a huge crater in the ground.

One side of the hotel collapsed entirely.

Vieira de Mello died after being trapped in the wreckage for five hours.

Authorities yesterday identified two more Americans among the victims: Arthur C. Helton, director of peace and conflict studies at the Council on Foreign Relations, and Martha Teas of the UN Office for the Coordination of Humanitarian Affairs. Richard Hooper, a New Yorker who worked for the UN's political affairs unit in Iraq, was identified on Tuesday.

FBI agents found the truck's identification number and its license plate yesterday. Yet tracking its original owner is going to be very difficult, Fuentes said.

Blanket looting of government ministries after Hussein's government collapsed on April 9 has stripped the country of millions of public documents, including vehicle-registration papers.

Other key pieces of evidence remained missing last night. Fuentes said the FBI had failed to find a single eyewitness who had seen the truck on the street before the explosion.

The blast shattered and charred the vehicle. Bits of human remains were found among the pieces yesterday, Fuentes said.

Perhaps the most crucial missing clue is the bomb's triggering device, which could suggest which group was responsible for the attack.

A "shaped charger" capable of sending the blast in a specific direction is a hallmark of some groups, including Ansar Al Islam, an organization loosely linked to Al Qaeda, which operated out of northern Iraq before the war.

Finding such a charger could suggest that Vieira de Mello was the bomb's specific target. Since arriving here in June, the 55-year-old Brazilian diplomat had earned widespread affection from Iraqis, who saw him as the person most capable of resuscitating their country after decades of war and dictatorship.

It was unclear last night how extensive the UN's evacuation would be. The UN agencies maintained Baghdad offices through 12 years of international sanctions and are involved in countless projects, from distributing food to building schools and purifying water.

Questions over US military guarding the United Nations facilities are entangled in months of tension between the two bodies. The Security Council did not endorse the war last March, and it accused Washington of circumventing the UN charter by going to war without its sanction.

Since then, UN officials have said that being guarded by American soldiers would compromise their neutrality. Scores of private relief organizations have opened offices in Baghdad, operating with no visible security.

Vieira de Mello had jogged alone along the Tigris River every Friday morning, according to employees at the Cedar Hotel, where he lived in Room 403.

"This is the first day I've seen American soldiers on this street," said hotel receptionist Nidal Mansour yesterday. "The Americans knew the UN was living here, but we had no security here."

Three American armored cars cordoned off the hotel's block yesterday, while UN staff members loaded suitcases into off-road vehicles and headed for the airport for flights to Jordan.

Inside the lobby, their colleagues sat stunned, some crying in small groups, as the extent of the tragedy sank in.

Ahmed al-Khashad, a UN security guard in the lobby, said: "The fault is the US Army. They knew that their duty was to protect us. They wait for our people to get killed, and now they come protect us."

Asked whether American soldiers had neglected UN protection, the UN spokesman in Iraq, Salim Lone, said yesterday: "It's too easy to say that. The Americans cannot even protect their own soldiers."

The US civilians running Iraq are cloistered in Hussein's old palace behind deep military cordons that block public vehicles about a half mile from the palace doors. By contrast, Tuesday's lethal truck drove along a public pathway open to all traffic. Security for UN workers today is markedly different than it was during the Hussein regime.

"In Saddam's time there were policeman posted on the roofs across the street from the building," said Fraidun Abdul Karim, 54, an electrical engineer who lives a few blocks from the bombed building. "There were many checkpoints. They didn't allow cars to stop there."

Outside the offices of the UN Children's Fund yesterday, two unarmed traffic police officers on motorcycles were the only visible security. An Iraqi doorman turned away a reporter, saying the expatriate staff had not come to work.

"There are many places the Americans should protect in Baghdad," said Nadeer Fadel, a manager at the Rimal Hotel, where about 10 World Bank employees have been living. "But that would take 1 million soldiers."

© Copyright 2003 Globe Newspaper Company.

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