The blast killed a U.S. soldier, a State Department official and a civilian contractor working for the Defense Department as their convoy headed through Fallujah to a nearby construction site on Monday, the military said. Two others were wounded.
Insurgents once held sway over the western province of Anbar, which was the scene of some of the deadliest fighting of the war. But violence fell off dramatically after Sunni fighters turned on al-Qaida in Iraq and joined U.S. forces in what has become known in Iraq as "the Awakening."
Insurgents, though, have continued to sporadically target American and Iraqi security forces in Fallujah, where four employees of the Blackwater security firm were ambushed in 2004 by insurgents and their remains strung from a bridge.
The U.S. military withdrew last year from most of the cities in the vast Anbar province, including Fallujah, well before a June 30 deadline for U.S. troops to leave Iraq's urban areas.
President Barack Obama has announced the withdrawal of U.S. combat troops from Iraq by Aug. 31, 2010, leaving 30,000 to 50,000 troops as advisers and trainers. Under an Iraqi-U.S. security pact, the remaining troops must leave by the end of 2011.
Like many cities in Iraq, Fallujah has a number of U.S.-supported reconstruction projects, many of them aimed at improving essential services and promoting businesses.
American military and government officials see the projects as essential to helping maintain security gains. Some of the projects are overseen by provisional reconstruction teams and a joint U.S. civil-military office. Others are directed by the State Department's U.S. Agency for International Development.
The deaths of State Department employees in Iraq have been rare. At least six other State Department employees have been killed in Iraq, including Steven Farley who was killed in June 2008 during a bombing at an Iraqi council building in Baghdad's Sadr City.
The military said the identities of those killed Monday were being withheld pending notification of next of kin.
As of Monday, at least 4,301 members of the U.S. military had died in the Iraq war since it began in March 2003.