It went off outside the UN's base in the Iraqi capital, and killed at least 17 UN members of staff and Iraqis as well.
A US military spokesman said they thought the explosion was a car or truck bomb - perhaps in a suicide attack.
De Mello, 55, died after being trapped in the rubble of the building for a number of hours.
Rescue workers are still searching for survivors in the rubble.
In recent weeks water and oil pipelines have also been attacked in Iraq.