American investigators looking into the suicide bombing of the United Nations compound on Tuesday are focusing on the possibility that the attackers were assisted by Iraqi security guards who worked there, a senior American official here said Tuesday.

The official said all of the guards at the compound were agents of the Iraqi secret services, to whom they reported on United Nations activities before the war. The United Nations continued to employ them after the war was over, the official said.

The official said that when investigators began questioning the guards, two of them asserted that they were entitled to "diplomatic immunity" and refused to cooperate. Diplomats working in foreign countries are often entitled to immunity from prosecution by local authorities, but the official said the two guards could make no such claim.

Investigators are continuing to question the guards, the official said.

"We believe the U.N.'s security was seriously compromised," the official said, adding that "we have serious concerns about the placement of the vehicle" and the timing of the attack. The bomb exploded directly under the third-floor office of the United Nations coordinator for Iraq, Sergio Vieira de Mello, while he was meeting with a prominent American human rights advocate, Arthur C. Helton. Both men were killed, along with several top aides to Mr. Vieira de Mello.

In New York, a United Nations official reacted skeptically to the assertions. "All of us are trying to get to the bottom of this," said Fred Eckhard, spokesman for the secretary general, Kofi Annan. "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."

No one connected to the United Nations office in Baghdad, which was demolished in the bombing, could be reached for comment. The United Nations had a large presence in Iraq before the war, running the oil-for-food program and housing teams of weapons inspectors.

The American official said investigators were trying to determine which, if any, of the guards failed to report to work the day of the attack. Even before the war, the government of Saddam Hussein was widely known to assign intelligence agents to guard and guide foreigners visiting or living in the country.

Suspicions have focused on the guards rather than other local United Nations personnel because their links to Mr. Hussein's security service were close. Under the former government, they had to report to the security service once a week on the activities of United Nations personnel, western officials said.

Even so, United Nations administrators retained the guards after Mr. Hussein's government was removed. American officials said earlier this week that the administrators had also turned down an American offer to provide greater security around the building.

The American official also questioned the wisdom of United Nations officials who ordered the construction of a cement wall around the compound. In some places, he pointed out, the wall was just 12 feet from the building.

Tensions have repeatedly flared between the United States and the United Nations over Iraq. The United Nations has been confined to a marginal role in Iraq since the war, and had sought to project a sympathetic and approachable image to the Iraqi people, partly by shunning the heavy protection surrounding American troops and installations here.

In the attack on Tuesday, the deadliest ever against the organization, a suicide bomber drove a flatbed truck loaded with 1,500 pounds of explosives up to the wall of the compound and set it off. Rescue workers pulled three more bodies out of the rubble today, bringing the total dead to 23.

At the scene of the bombing, a green military backhoe lifted great chunks of the rubble and dropped them into dump trucks to spread it out on the ground for later examination. The work paused momentarily to allow two dogs from a Turkish cadaver team to search the wreckage for missing bodies or body parts. None were found.

Throughout the day, United Nations staffers came to gape at the wreckage of what had been their offices and to try to salvage the hard drives of their computers. Looking at the remnants of the office of Mr. Vieira de Mello, one woman gasped: "How could they have left this place so unprotected?"

The possibility that Iraqi security guards had cooperated in the bombing increased suspicions that Mr. Vieira de Mello was a target of the attack, the American official said. The truck pulled up to the wall just below his office while he was inside meeting with other American officials.

"We are very concerned about the possibility" that Mr. De Mello was chosen as a target, the official said.

The official said that the revelation that former agents of Mr. Hussein were still working at the compound had also added to their suspicions that it was loyalists to the deposed president who carried out the attack.

Investigators said yesterday that they were intrigued by the discovery that the explosive device used in the attack consisted of an array of munitions, including mortar shells, hand grenades and a Soviet-made 500-pound bomb.

Iraqi munitions, looted from storehouses and dumped by army veterans, were widely available in the days after the war. Many munitions storage sites, like so many other government facilities here, were looted in the chaos that followed the removal of Mr. Hussein's government. So many weapons and munitions flooded the streets that they could be purchased at prices well-below cost.

The American official said the investigators had determined that the Russian-made truck was not registered with the Iraqi government, meaning that it may have entered the country after the war. The Israeli ambassador to the United Nations, Dan Gillerman, said today in New York that the truck had entered the country from Syria.

The American official said that investigators had recovered the vehicle identification number for the truck and were trying to trace it back to its owner.

Also today, a previously unknown Iraqi group called the Armed Vanguards of the Second Muhammad Army had claimed responsibility for the attack in a statement sent to an Arabic television channel, Al Arabiya.

"The statement promised to make war on all foreigners," the station said.

"It warned Arab countries against sending armed forces to Iraq and called for continuing what it called acts of jihad against all those who help the Americans, even if they are Arab or Muslim."

There was no way to verify the authenticity of the statement. But in the days since the bombings, some Iraqi officials have raised the possibility that loyalists of Mr. Hussein are now collaborating with radical Islamic groups to attack Western targets. Mr. Hussein is believed to have access to dozens of safe houses in Baghdad and areas north and west of the capital.

A senior Iraqi official said this week that recent intelligence suggested that Mr. Hussein's loyalists had made their money and hideouts available to Islamic militant groups like Ansar al-Islam, which American officials believe has been plotting attacks against Western targets in Baghdad.

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