It is thought a suicide bomber parked a lorry outside the offices
FBI investigators are helping to search through the rubble of the devastated UN building in Baghdad for clues as to who carried out Tuesday's bombing.
A number plate believed to be from the lorry used by the attackers was found 500 metres from the site, and its registration details are now being tracked down.
FBI special agent Thomas Fuentes said the explosives appeared to have come from Saddam Hussein's pre-war arsenal.
And he said human remains found in the area where the bomb exploded suggested a suicide bombing but warned that testing was needed to confirm this.
At least 20 people were killed - including the UN envoy in Iraq, Sergio Vieira de Mello - in the blast.
Paul Bremer, chief US administrator in Iraq, told US television the blast was "of a size not seen before".
He told the ABC channel it was "not yet connected" to the Jordanian embassy attack in Baghdad two weeks ago but said they were both "acts of disciplined people".
The FBI said the UN bomb was made from 454 kilograms (1,000 lbs) of old munitions including one single 226-kg (500-lb) bomb.
"We believe that the bomb was made using the army stock of explosives," said Special Agent Fuentes.
"I cannot say that its manufacture required great knowledge or efforts."
He added that it was not yet established whether the bomb had been made by Saddam Hussein followers or activists from another country.
Paul Bremer has highlighted three groups of suspects - Baathist supporters of Saddam Hussein, the Iraqi Ansar al-Islam or foreign Islamist militants.
Saudi Islamists may be in Iraq after May's suicide bombings in Riyadh
Post-war Iraq has become a magnet for Islamists looking for a new arena in which to wage a jihad, or holy war, against America, says the BBC's Middle East analyst, Roger Hardy.
He says the country is relatively easy to enter, and has reportedly attracted Saudi Islamists escaping a crackdown in their own country as well as members of the Ansar al-Islam group suspected of carrying out the Jordanian embassy bombing.
But, says our correspondent, it is not only radical Islamists who are hostile to the UN for giving what they see as a false legitimacy to US foreign policy.
Many Iraqis deeply resented the stringent UN sanctions they suffered throughout the 1990s, and saw the work of UN weapons inspectors as an infringement of their country's sovereignty.
Although the immediate target is the UN, the attack is designed to send a message to the United States - that the country is ungovernable and the Americans are unable to protect themselves or anyone working them, says our correspondent.
UN HEADQUARTERS BOMBING
1. Truck seen on access road next to UN compound before it exploded.
2. Explosion destroyed three storeys, including office of Sergio Vieira de Mello
3. Explosion left crater nearly five metres across and two metres deep