Marines accused of shooting and killing civilians after a homicide bombing in Afghanistan are under U.S. investigation, and their entire unit has been ordered to leave the country, officials said Friday.

Army Maj. Gen. Francis H. Kearney III, head of Special Operations Command Central, ordered the unit of about 120 Marines out of Afghanistan and initiated an investigation into the March 4 incident, said Lt. Col. Lou Leto, spokesman at Kearney's command headquarters in Tampa, Fla.

It is highly unusual for any combat unit, either special operations or conventional, to have its mission cut short.

A spokesman for the Marine unit, Maj. Cliff Gilmore, said it is in the process of leaving Afghanistan, but he declined to provide details on the timing and new location, citing a need for security.

In the March 4 incident in Nangahar province, an explosives-rigged minivan crashed into a convoy of Marines that U.S. officials said also came under fire from gunmen. As many as 10 Afghans were killed and 34 wounded as the convoy made an escape. Injured Afghans said the Americans fired on civilian cars and pedestrians as they sped away.

U.S. military officials said militant gunmen shot at Marines and may have caused some of the civilian casualties.

President Hamid Karzai condemned the incident, which was one among several involving U.S. forces in which civilians were killed and injured.

Leto, the spokesman at Special Operations Command Central headquarters, said the Marines, after being ambushed, responded in a way that created "perceptions (that) have really damaged the relationship between the local population and this unit."

Therefore, he said, "the general felt it was best to move them out of that area."

Gilmore said the Marine company would complete its overseas deployment with the 26th Marine Expeditionary Unit, which is the larger unit it sailed with from Camp Lejeune, N.C., in January, but it will no longer operate in Afghanistan.

Of the four Marine Special Operations Command companies that have been established since the command was created in February 2006, the one ordered out of Afghanistan was the first to deploy abroad, Gilmore said. By September 2008 there are to be nine companies operating as part of two special operations battalions, he said.

For years the Marines resisted creating special operations units, arguing that would run counter to their philosophy of viewing all Marines as elite fighters and not singling out elements as special. But former Defense Secretary Donald H. Rumsfeld pressed them to establish a separate command — the Marine Special Operations Command — to train and equip forces for the multi-service Special Operations Command.

There are about 25,000 U.S. troops in Afghanistan, mostly conventional combat forces and support units.