A rocket-propelled grenade slammed into a U.S. military ambulance Thursday, killing three American soldiers, the latest in a series of attacks on U.S. personnel or their offices. The ambulance was transporting a wounded American soldier to a medical facility when it came under fire on a highway about 20 miles south of Baghdad. Three mortar shells exploded Tuesday outside a coalition-run humanitarian aid office in the town of Samarra, north of Baghdad, killing an Iraqi bystander and wounding 12 others, hospital officials and U.S. officers in the town said Thursday. No American forces were hurt. The military initially said the attack happened Wednesday. Attackers also fired a rocket-propelled grenade that struck a U.S. tank in Samarra, said Sgt. Steven Stoddard with the Army's 4th Infantry Division. Another tank fired back, killing one attacker, while the second was captured, Stoddard said. In west Baghdad, an Army truck was hit by what witnesses said was a rocket-propelled grenade. The torn-apart truck sat burning on the edge of the highway. Witnesses said there were casualties, but U.S. military police at the scene said the vehicle broke down earlier and was set on fire after being left alone while soldiers prepared to remove it. The mortar rounds in Samarra, 75 miles north of Baghdad, exploded outside the Civil Military Operations Center. U.S. soldiers heard three explosions and asked local police to investigate, said a U.S. Central Command statement. Samarra police found the injured and killed and that soldiers were unable to find the attackers, the statement said. The office coordinates between the military and civilian agencies in the area. **"Death to Bush!"*** Meanwhile, scores of angry mourners fired Kalashnikov assault rifles into the air and shouted curses at the United States during a procession Thursday for two Iraqis who were shot dead by U.S. troops at a protest by disgruntled former army officers. Shouting "Death to Bush!" and "Revenge!," mourners marched with the body of 32-year-old former Iraqi army officer Tareq Hussein Mohammed, killed by U.S. troops, from his house in northern Baghdad to a mosque. Mohammed was one of two men shot outside the gate of the coalition headquarters in Baghdad during a demonstration of ex-soldiers demanding their salaries. The men were shot after the protest turned violent, the U.S. military said. "Iraqis are going to kill Americans. We are going to take revenge for Tareq's blood," said Salwa Mohammed, a relative of the slain man. As the U.S. military grappled with an increase in guerrilla attacks, the United Nations reported that an increase in power outages in the capital of Baghdad was caused by sabotage to Iraqi power lines. The United Nations also reported that humanitarian assistance vehicles were being fired upon, along with those of the American military. Meanwhile, the U.S. Army's Baghdad radio station began broadcasting appeals for Iraqis - including ex-military personnel - to join the civilian police force in Baghdad and Fallujah. Some say the upsurge in violence is at least partly due to the huge number of former soldiers and officers of the ousted regime who lost their jobs. Iraqi cities have been on edge since Sunday, when coalition forces began house-to-house searches in Baghdad for banned weapons and suspected activists trying to undermine the U.S.-led occupation. U.S. forces said Wednesday they captured Saddam Hussein's top aide and presidential secretary, a man who American officials believe knows the fate of the deposed Iraqi leader and has information about banned weapons. Abid Hamid Mahmud al-Tikriti was No. 4 on the U.S. most-wanted list of Iraqi leaders, behind only Saddam and sons Qusai and Odai. **PHOTO CAPTION*** Iraqi boys climb on top of a destroyed US military ambulance in the al-Dura district of Baghdad. (AFP/Cris Bouroncle)