Since Sept 11, 2001, there have been numerous terrorist attacks, like Beslam, London, Madrid, the resort in Kenya, attacks in Egypt, attacks in Iraq (by Al-Qaida, and other groups that don't distinguish between non-combatants and 'liberators/occupators).
However, on August 19th, 2003 an explosion rocked Baghdad, now known as the Canal Hotel bombing. This bombing was directed at the United Nations and killed the UN representative to Iraq, and also the United Nations High Commisioner for Human Rights, the same person, Sergio Vieira de Mello. The explosion was carried out by a suicide bomber, who smashed his way through the fence guarding the compound with a cement truck.
Sérgio Vieira de Mello, 55 (Brazil): UN Secretary-general's special Iraqi envoy.
Saad Hermiz Abona, 45 (Iraq): working for UN contracting firm
Renam Al-Farra, 29 (Jordan): an employee of the UN Office for the Coordination of Humanitarian Affairs
Raid Shaker Mustafa Al-Mahdawi, 32 (Iraq): United Nations Monitoring, Verification and Inspection Commission (UNMOVIC)
Emaad Ahmed Salman Al-Jobary, 45 (Iraq): Electrician for UNMOVIC
Omar Kahtan Mohamed Al-Orfali, 34(Iraq): Driver
Leen Assad Al-Qadi, 32 (Iraq): UN Office for the Coordination of Humanitarian Affairs in Iraq (UNOHCI)
Ranillo Buenaventura, 47 (Philippines): UNOHCI
Gillian Clark, 47 (Canada): Christian Children's Fund
Arthur Helton, 54 (United States): director of peace and conflict studies at the US Council on Foreign Relations.
Richard Hooper, 40 (United States): UN Department of Political Affairs.
Reza Hosseini, 43 (Iran): employed by UN Office for the Humanitarian Coordinator in Iraq
Ihsan Taha Husein, 26 (Iraq): UN driver for office of project services
Jean-Selim Kanaan, 33 (Egypt): Member of Vieira de Mello's staff.
Chris Klein-Beekman, 32 (Canada): UN Children's Fund's program coordinator.
Manuel Martín-Oar, 56 (Spain): naval captain, assistant to the Spanish special ambassador to Iraq
Khidir Saleem Sahir, (Iraq): Civilian
Alya Souza, 54 (Iraq): worked for the World Bank
Martha Teas, 47 (United States): manager of UN humanitarian coordination office
Basim Mahmoud Utaiwi, 40 (Iraq): Security guard for UNOHCI
Fiona Watson, 35 (Britain): Member of Vieira de Mello's staff
Nadia Younes, 57 (Egypt): Chief of Staff for Vieira de Mello
Militant Sunni branch of Islam followers
Militant Shiite branch of Islam followers
Iraqis angered by Coalition occupiers
Iraqis with tribal loyalties
Foreign agents (such as al-Qaida)
Members of Ansar al-Islam group
The suicide bombing of the United Nations in Baghdad drew overwhelming condemnation.
Kofi Annan, United Nations Secretary-General, commented that the bombing would not stop the organization's efforts to rebuild Iraq, and said: "Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq for one purpose only: to help the Iraqi people recover their independence and sovereignty, and to rebuild their country as fast as possible, under leaders of their own choosing."
However, a conspiracy theory that has not been ruled out is that it was the United States, or 'friendly elements to the United States that committed this crime'. Why, because a man connected to the Oil-for-food scandal, Benon Sevan, was doing a press conference in regards to the scandal. However, there was no US press there, only four journalists, three arabs, and a Japanese reporter, could the US reporters been warned about a strike at the UN. It was getting more involved in Iraq and wanted more involvement in Iraq and Iraqi affairs, so who knows.
I don't subscribe to this theory myself, but I do not know for sure, because no one has been arrested for the crime. And the Bush Administration has a well known disdain for the UN.
I will leave it up to you to decide.