Insurgents fired at least six mortar rounds toward a U.S. military base Thursday, but hit a nearby bus station instead, killing four Iraqis and wounding 21, officials said.

None of the shells struck the base near Musayyib city, which is 40 miles south of Baghdad (search), the U.S. military said.

Police Capt. Muthana Khalid (search) said a total of 12 mortars were fired and that most hit the bus station during the morning commute.

U.S. forces sent a five-man medical team to the bus station, including a doctor, to help the wounded, and Iraqi forces brought medical supplies, the U.S. military said in a statement. One seriously wounded civilian was airlifted to a U.S. hospital; the others were treated at a local hospital, the U.S. military said.

In another attack Thursday, a homicide car bomb exploded near an Iraqi army checkpoint, wounding four Iraqi soldiers, three U.S. soldiers and seven Iraqi civilians, the U.S. military said.

The attack occurred outside Tikrit (search), which is 80 miles north of Baghdad, said U.S. Maj. Richard Goldenberg. He said the injuries the U.S. soldiers suffered were not life threatening.

In the capital, Lt. Col. Ala'a Khalil Ibrahim, who worked in the visa section of the Interior Ministry, was shot dead on the way to work by gunmen in Baghdad's eastern section of al-Shaab, police officials said on condition of anonymity.

On Thursday, Bob Callahan (search), a spokesman for the U.S. Embassy in Baghdad, offered its condolences to the family of Lamia Abed Khadouri al-Sakri, 50, a Shiite Muslim legislator in the National Assembly.

She was shot and killed at her home in Baghdad on Wednesday, becoming the first elected official slain since millions of Iraqis voted in the country's landmark election for parliament on Jan. 30

"We offer our condolences to her family. We condemn this cowardly attack on a woman who was working to bring democracy to Iraq, and we admire her courage and her dedication to making life better in Iraq," Callahan said in a statement.