March 24, CNN

BAGHDAD, Iraq -- Four U.S. soldiers died in a roadside bombing in Iraq on Sunday, military officials reported, bringing the American toll in the 5-year-old war to the grim milestone of 4,000 deaths. Eight of those killed were civilians working for the Pentagon.

The four were killed when their vehicle was hit by an improvised explosive device while patrolling a neighborhood in southern Baghdad, the U.S. military headquarters in Iraq reported Sunday night. A fifth soldier was wounded in the attack, which took place about 10 a.m. (3 a.m. ET).

The U.S. milestone comes just days after Americans marked the fifth anniversary of the start of the war.

Meanwhile, estimates of the Iraqi death toll range from about 80,000 to the hundreds of thousands, with another 2 million forced to leave the country and 2.5 million people displaced within Iraq, according to the U.N. High Commissioner for Refugees.

President Bush ordered U.S. troops into Iraq on March 19, 2003, after months of warnings that then-Iraqi leader Saddam Hussein was concealing stockpiles of chemical and biological weapons and efforts to build a nuclear bomb.

U.N. weapons inspectors found no sign of banned weapons before the invasion, and the CIA later concluded that Iraq had dismantled its weapons programs in the 1990s.

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photo: A U.S. soldier secures the site of a suicide car bombing Sunday in Baghdad. (AFP/Getty Images)