As the U.S. military reported two more American troop deaths on Friday, investigators were looking into whether Tuesday's bombing of U.N. headquarters in Baghdad may have been an inside job.
Six other U.S. soldiers were wounded Friday when their truck ran over a roadside bomb about 125 miles north of Baghdad.
Investigators at the Canal Hotel (search), which was nearly demolished in Tuesday's blast, said former Iraqi intelligence agents working as guards in the compound may have assisted the attackers.
U.S. soldiers and Iraqi police worked together, scouring for human remains in the rubble, as 86 seriously wounded U.N. workers waited to be airlifted out of Iraq for medical care abroad. The blast killed at least 23 people.
The security guards at the U.N. headquarters had been selected by Saddam Hussein's government before the war, and had reported on the movements of U.N. staffers, especially weapons inspectors, in and out of the compound, a U.S. official in Baghdad said.
The United Nations continued to employ the guards during and after the war.
"We believe the U.N.'s security was seriously compromised," a senior U.S. official told The New York Times, adding that "we have serious concerns about the placement of the vehicle" and the timing of the attack.
A State Deptartment official told Fox News that the "inside job" theory is not a working theory for U.S. investigators, including the FBI.
However, Iraqi police are in charge of the investigation, and whether they have this as a working theory is not known and remains a possibility, the official said.
The bomb exploded directly under the third-floor office of the United Nations coordinator for Iraq, Sergio Vieira de Mello (search), as he was meeting with a prominent American human rights advocate, Arthur C. Helton. Both men were killed, along with several top aides to Vieira de Mello.
Former New York police commissioner Bernard Kerik (search), who is in charge of Iraqi security forces, told The Associated Press that there are "concerns about some of the people who were working there."
He said some of the Iraqi personnel initially refused to cooperate with authorities and were being interrogated.
"We want to know who was in the building, vendors, anyone who had to make deliveries, anyone who had to be in the building for any other reason," Kerik said.
Investigators are checking to see if any guards failed to report for duty on Tuesday.
U.N. officials said they won't increase the number of U.S. soldiers standing guard outside its facilities from the dozen or so it had before the attack.
"We will always remain a soft target … we are conscious of that, but that is the way we operate. We are an open organization," said Ramiro Lopes da Silva, Iraq coordinator for U.N. humanitarian programs.
Some experts argue that's just not good enough.
"Of course, it's an enormous security risk — people who were members of Saddam Hussein's intelligence corps acting in any security capacity," Rep. Adam Schiff, D-Calif., told Fox News. "We have to recognize that the tactics have changed, they [attacks] have escalated."
Gregory Esslinger, a former FBI terrorism special agent, pointed out that homicide bombings usually are well planned and take a number of people and pieces of good intelligence to pull off.
"It's very possible the bomber or bombers had inside information," Esslinger told Fox News. "I think it's something that has to be taken very seriously."
But a U.N. official in New York was skeptical of the "inside job" theory, the Times reported.
"All of us are trying to get to the bottom of this," said Fred Eckhard, spokesman for U.N. Secretary-General Kofi Annan (search). "In fact, the secretary-general is sending his security coordinator to Baghdad this evening to investigate the bombing. But the task is not made easier by the conspiracy theories circulating. We'll have to separate as best we can fact from speculation."
A previously unknown group calling itself the "Armed Vanguards of a Second Muhammed Army" (search) claimed responsibility for the homicide truck attack. The group pledged "to continue fighting every foreigner [in Iraq] and to carry out similar operations" in a fax to Arab media Thursday.
Also on Thursday, the Israeli ambassador to the United Nations said intelligence reports indicated that the truck used in the bombing was from Syria's capital. Damascus denies any links to the truck.
Gen. John Abizaid (search), the head of U.S. Central Command, said Thursday that as the number of 55-most-wanted Iraqis are hunted down by the coalition and either killed or captured, the No. 1 threat to coalition forces in the country is terrorism. Foreign fighters coming in to Iraq from countries such as Syria add to the problem, he said.
Wanted: More International Help
The United States, meanwhile, has made a new push for a U.N. resolution calling on nations to send assistance to help American forces in Iraq to increase security and make the force more international.
But U.S. Secretary of State Colin Powell, who met with U.N. Secretary-General Kofi Annan Thursday, said Washington wouldn't surrender any control in Iraq — a major sticking point to convincing other countries to sending troops.
Powell noted that 30 nations already are providing 22,000 troops — 11,000 of them from Britain.
"But perhaps additional language and a new resolution might encourage others," he said.
Annan said the Security Council might consider a "U.N.-mandated multinational force."
But that would require more than "just burden-sharing but also sharing decision and responsibility."
But he added that it would be "very difficult" to pass a resolution offering more troops without the U.N. gaining some oversight power and having a say in decisions.
Washington's preference is to boost the Iraqi police force, but it wants more international help supporting those forces and training them.
Two U.S. Soldiers Die
The U.S. military announced the deaths of two U.S. soldiers Friday.
One serviceman was killed in action on Thursday in al Hilla, 34 miles south of Baghdad, said Spc. Margo Doers. The second casualty was from the 1st Armored Division based in Baghdad.
Thursday's deaths brought the number of U.S. combat deaths in Iraq to 179, 32 more than in the first Gulf War.
Since President Bush declared an end to formal combat on May 1, 135 U.S. soldiers have died in Iraq, including combat deaths. In all, 273 U.S. soldiers have died of all causes since the beginning of military operations in Iraq.
Six U.S. soldiers were also wounded Friday when their 5-ton truck ran over a roadside bomb on the outskirts of Baiji, about 125 miles north of Baghdad.
The victims were evacuated in a helicopter to an Army field hospital north of Tikrit (search), where one was in critical condition awaiting surgery. The others were in stable condition.
U.S. troops also shot and killed one Iraqi who opened fire with an AK-47 rifle at a patrol early Friday.
The United Nations was temporarily moving about 100 support and administrative staff — out of a total work force of 300 — to Jordan and Cyprus.
Two U.N. employees were still unaccounted for and an unknown number of people — visitors to the building — were still buried in the rubble.
But a Filipino woman who was reported to have been killed in the bombing is still alive, family members in New York City and a Philippine official said. U.N. staffer Marilyn Manuel was being treated at a military hospital outside Baghdad.
The Associated Press contributed to this report.