The deadly bomb that ripped through U.N. headquarters in Baghdad was made from 1,000 pounds of old Soviet-made munitions from Saddam Hussein's arsenal that didn't take a genius to figure out how to make, the FBI said Wednesday.

But FBI (search) investigators said it was impossible yet to say whether Tuesday's unprecedented truck-bomb attack on the world organization was the work of Saddam loyalists or foreign terrorists.

"We believe it (the bomb) was made from existing military ordnance. ... I cannot say that it required any great degree of sophistication or expertise to create," said FBI special agent Tom Fuentes.

Meanwhile, hopes of finding survivors in the destruction faded Wednesday afternoon, 24 hours after an explosives-rigged truck brought down the facade of U.N. offices in the Canal Hotel (search), killing at least 20 people including the top U.N. envoy in Iraq, Sergio Vieira de Mello (search). About 100 people were wounded.

L. Paul Bremer (search), the top civilian administrator in Iraq, said Wednesday that it has not been determined for sure if the bombing was a homicide attack.

Bremer also said U.S. intelligence believes more than 100 foreign terrorists are in Iraq, some of whom have used passports and travel documents from Syria, Sudan and Yemen.

Some are members of Ansar al-Islam (search), an Al Qaeda (search)-linked terror group considered by the United States "to be one of the world's more dangerous groups," Bremer said in a television interview.

U.N. Secretary-General Kofi Annan (search) vowed the deadly bombing would not drive the world organization out of Iraq.

"We will persevere. We will continue. It is essential work," Annan said at a news conference in Stockholm, where he stopped briefly before heading to U.N. headquarters in New York. "We will not be intimidated."

However, the World Bank (search) and the International Monetary Fund (search) pulled their staff out of Iraq on Wednesday after the bombing of the U.N. headquarters in Baghdad, a U.S. official said.

The IMF and World Bank are expected to provide billions of dollars in loans to help restart the country's economy and banking system.

U.N. spokesman Fred Eckhard said only two people among the 300 or so U.N. international staffers in Baghdad decided to leave the country.

"I think the staff is pretty committed to staying on the job," he said. "The leadership is reassessing security."

Fox News has learned the FBI is tracing a bumper and license plate found near the vehicle used in the bombings and believed to be from the actual truck bomb. The FBI also found body parts in the vehicle, leading investigators to further believe it was a homicide bombing.

Investigators also have the vehicle identification number of the truck and the engine number, Fuentes said. Department of Motor Vehicle records were on computers that were taken from agency employees at the fall of the Iraqi regime, Fuentes said.

That, combined with the fact there was so much looting of government property at Baghdad's fall, will make it hard to track down the perpetrator based on where the truck came from.

"It's going to be tough, especially if the vehicle belongs to the military or one of the ministries during the regime," Fuentes said. "Merely identifying the vehicle will not in itself give us all the answers."

Fuentes said at least 1,000 pounds of military-grade explosives, including a single 500-pound bomb, were placed on a KAMAZ flatbed truck to be detonated. They were materials from Saddam Hussein's prewar arsenal that weren't hard to assemble. Those trucks were made in the Soviet Union.

Investigators also discovered remnants of Soviet-manufactured munitions such as artillery shells and an unexploded grenade at the scene.

"This would be in large supply here during the regime ... these are items that were manufactured in some cases, 20 to 30 years ago," Fuentes said, pointing authorities toward the deduction that perhaps the perpetrators were local.

"They could be [local] but the list of potential suspects is a huge list," Fuentes stressed.

Bernard Kerik, the former New York City police commissioner who is rebuilding the Iraqi police force, said it was "much too early" to say if Usama bin Laden's Al Qaeda network was behind the attack. "We don't have that kind of evidence yet."

Iraqi Governing Council (search) member Ahmad Chalabi (search) said the council received information Aug. 14 that there would be a terror attack in Baghdad, and the council warned the United States.

"The information said that the attack would be aimed at a soft-target, not the American military or forces," he told reporters. "The information said the attack would use a truck and would be carried out by using a suicide mechanism or by remote control. We shared this information with the Americans."

Some nations raised fears of more attacks; others suggested Washington end its occupation of Iraq. China's President Hu Jintao urged the United Nations to continue its mission to rebuild the nation, and Germany's Chancellor Gerhard Schroeder blamed the attack on "forces that do not want the rebuilding of Iraq to take place in peace and freedom."

Others presumably dead include: Rick Hooper of the United States; Ranillo Buenaventura and Marilyn Manuel of the Philippines; Jean-Selim Kanaan of Egypt; Fiona Watson of the United Kingdom and Christopher Klein-Beekman, program coordinator for the UN Children’s Fund (UNICEF (search)) in Iraq, who was a 32-year-old Canadian.

Tuesday's bomb blasted a 6-foot-deep crater in the ground, shredding the facade of the Canal Hotel, which houses U.N. offices.

Except for a new concrete wall built recently, U.N. officials at the headquarters refused heavy security because the U.N. "did not want a large American presence outside," said U.N. spokesman Salim Lone.

Annan said Wednesday that both the United States and the United Nations made mistakes when it came to security in Iraq.

"The coalition has made some mistakes and maybe we have, too," Annan said from New York. "I don't want to get into finger pointing but along the way mistakes have been made by all concerned."

The U.N. temporarily suspended operations; travel arrangements were being made for employees wanting to leave the country.

Council members think the bombing was committed by members of Saddam's regime with the help of foreign militants.

"There is a feeling, based on accumulated data from the past, that it is the remnants of Saddam's regime and their friends (behind the attack)," said Chalabi, indicating he was including Al Qaeda.

The bombing came nearly two weeks after a similar homicide car bomb killed 19 people at the Jordanian Embassy in Baghdad.

The U.S. military warned of foreign Islamic militants slipping into the country and has said Ansar al-Islam was a possible suspect in the Jordanian Embassy bombing.

Bremer said he does not believe the bombing is connected to the recent acts of sabotage on Iraq's so-called "soft targets" with little security.

"They appear to be the acts of at least disciplined people," Bremer said in a television interview. "Whether they're centrally coordinated has not yet been shown."

In a fresh attack on American forces Wednesday, a soldier was killed and another wounded after they came under small arms fire and crashed into another vehicle while traveling in a convoy near Diwaniyah, 100 miles south of Baghdad, the military said.

Also, insurgents fired a rocket-propelled grenade at a U.S. convoy in Tikrit, killing a civilian working for the occupation force and injuring two soldiers, U.S. Maj. Bryan Luke said.

Fox News' Steve Harrigan and The Associated Press contributed to this report.