"Even the leader of Iraq's main Shiite political alliance said he thought Zalmay Khalilzad, the American ambassador to Iraq, bore some responsibility. The Shiite leader, Abdul Aziz al-Hakim, said Mr. Khalilzad's veiled threat on Monday to withdraw American support if Iraqis could not form a nonsectarian government helped provoke the bombing. "This declaration gave a green light for these groups to do their operation, so he is responsible for a part of that," Mr. Hakim said at a news conference."