The bombing took place in Sadr City, a Shiite slum in the eastern part of the capital.
One of the U.S. civilians was an official with the State Department and the other worked for the Defense Department, U.S. embassy officials said. Ten people were wounded, including one U.S. soldier and three members of the Sadr City Council.
The blast occurred at 9:30 a.m., before a meeting to elect a chairman of the local council in the Shiite militia stronghold, an Iraqi official told the Associated Press, speaking on condition of anonymity for security reasons. The bomb exploded in the office where the meeting was to have occurred, the official said.
U.S. troops captured a suspect who tested positive for explosive residue after fleeing the scene, the military told the AP. The military blamed Shiite extremists for the attack.
"This was the fourth meeting of this district council, led by hardworking Iraqis determined to make a difference and set Sadr City off on the right path. 'Special Groups' are afraid of progress and afraid of empowering the people," U.S. military spokesman Lt. Col. John Digiambatista told the AP.
"Special Groups" is a term the U.S. military uses for Shiite militiamen refusing to follow a cease-fire order by anti-U.S. cleric Muqtada al-Sadr. The U.S. military commander in Iraq, Gen. David Petraeus, has called the "Special Groups" the greatest long-term threat in Iraq.
The explosion occurred a day after a gunman opened fire on U.S. soldiers attending a municipal council meeting southeast of Baghdad, killing two and wounding three. An interpreter was also killed.
The Iraqi military, meanwhile, pressed forward with an offensive against al-Sadr's Mahdi Army militia and other armed groups in the Maysan province. The Iraqi Defense Ministry announced a three-day deadline for all parties to voluntarily evacuate government buildings in the province, or face removal by force.
A provincial government official also said Iraqi security forces had begun a campaign to remove all portraits and pictures of senior religious figures from walls, buildings and the streets in the province as raids continued.