Questioning the guards has become a priority because of their ties to Saddam's intelligence apparatus, the official said. He said investigators were checking to see if any guards failed to report for duty on Tuesday.
Bernard Kerik, a former New York police commissioner in Iraq to help train police there, said there are "concerns about some of the people who were working there."
Kerik said some initially refused to cooperate with authorities and were being interrogated.
Adding to the insider theory is the location and timing of the explosion. The bomb went off near a top-level meeting that had just gotten under way.
In other developments:
U.S. soldiers and Iraqi police worked together searching for human remains in the rubble of the bombed U.N. headquarters, 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.
A previously unknown group claimed responsibility for the suicide attack. The group calling itself the "Armed Vanguards of a Second Muhammed Army" pledged "to continue fighting every foreigner (in Iraq) and to carry out similar operations" in a statement sent to the Dubai-based Al-Arabiya satellite channel. There was no way to verify the authenticity of the claim.
The United Nations' security guards were selected by Saddam Hussein's regime before the war and reported on the movements of U.N. staff at the Canal Hotel compound, which served as a base for weapons inspectors. The United Nations continued to employ the guards after the war.
The United Nations, despite the bombing, will not increase the number of U.S. soldiers standing guard outside its facilities from the dozen or so it had before the attack, said Ramiro Lopes da Silva, Iraq coordinator for U.N. humanitarian programs.
"It's not that we have anything against the coalition forces, but you do realize the presence of coalition forces does intimidate some of the people we need to speak to and work with," he told reporters at the blast site.
"We will always remain a soft target," he said. "We are conscious of that, but that is the way we operate. We are an open organization."
As the United Nations tried to recover its footing in Iraq after the bombing, the United States made a new push for a U.N. resolution calling on nations to send troops to help American forces in Iraq.
Secretary of State Colin Powell launched the drive for a new U.N. resolution on Thursday, calling on member states "to do more" to help Iraq. U.S. Ambassador John Negroponte said later that Washington wants the resolution to encourage countries to provide troops, money and help with police training.
France, Russia, India and Germany have ruled out sending soldiers to Iraq unless a multinational force is authorized by the United Nations. Without U.S. agreement to cede some control to the world body, diplomats said the possibility of a robust international force appeared unlikely to attract much new support.
Expressing concern at the increase in acts of terrorism and sabotage, France's deputy U.N. ambassador Michel Duclos asked whether "we would be in this state" if a genuine international partnership had been established at the outset, with U.N. guidance.
Powell reaffirmed the U.S. determination to succeed in Iraq and insisted U.S. leadership provides "competent control" of the coalition force.
Annan has ruled out a U.N. peacekeeping force for Iraq but backed a multinational force.
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.