The US is blaming terrorists and remnants of Saddam Hussein's former regime for the "horrific attacks" on four American contractors whose mutilated and burned bodies were dragged through the streets of the Iraqi city of Fallujah.
"It is offensive, it is despicable the way these individuals have been treated," White House press secretary Scott McClellan said.
McClellan said "the best way to honour those that lost their lives" was to continue with efforts to bring democracy to Iraq .
US State Department spokesman Adam Ereli said the contractors, all men, "were trying to make a difference and to help others".
US officials did not identify the dead or the nature of their work because the next of kin had not yet been notified.
In Fallujah today, shops and schools were open and there was no sound of any fighting a day after the frenzied crowds burned, mutilated and dragged the bodies through the streets and strung two of them up from a bridge after rebels ambushed their vehicles.
A handful of Iraqi police manned their standard roadside checkpoints and there was no sign of US troops.
Some of the bodies of the four Americans were loaded on to the back of a donkey-pulled wooden cart last night and paraded through Fallujah's streets as crowds clapped and whistled. It was not clear where the bodies were early today.
The abuse and mutilation of the contractors' corpses was similar to the scene more than a decade ago in Somalia, when a mob dragged corpses of US soldiers through the streets of Mogadishu, eventually leading to the American withdrawal from the African nation.
In Baghdad, US Brigadier General Mark Kimmitt said the coalition would not be deterred from its mission to rebuild Iraq , and that numerous reconstruction projects were moving forward nationwide even though attention was focused on the attacks.