A U.S. Marine unit broke international humanitarian law by using excessive force during a shooting spree last month that left 12 people dead, an Afghan human rights group said in a report Saturday.

The troops fired indiscriminately at pedestrians, people in cars, public buses and taxis in six different locations along a 10-mile stretch of road in Nangahar province after an explosives-rigged minivan crashed into their convoy on March 4, according to the report by Afghanistan's Independent Human Rights Commission.

Six people were killed near the blast site, while the other six died on the road as the troops sped away, said Ahmad Nader Nadery, the group's spokesman.

The dead included a 1-year-old boy, a 4-year-old girl and three women, the report said. Thirty-five people were wounded in the shootings.

"In failing to distinguish between civilians and legitimate military targets the U.S. Marines Corps Special Forces employed indiscriminate force," the report said. "Their actions thus constitute a serious violation of international humanitarian law standards."

The group said its report was based on interviews with victims and their families, witnesses, local community leaders, hospital officials and police.

A U.S. military commander has also determined that the Marines used excessive force and referred the case for possible criminal inquiry, a senior U.S. defense official told The Associated Press on Wednesday.

U.S. military officials said after the incident that the suicide attack was part of an ambush that included militant gunmen shooting at Marines, which may have caused some of the civilian casualties.

The human rights group's report said "there is some evidence at the immediate site of the incident to support this claim, but it is far from conclusive and all witnesses and Afghan government officials interviewed uniformly denied that any attack beyond the initial (suicide car bombing) took place."

The group also alleges that U.S. troops serving with NATO's International Security Assistance Force in southern Afghanistan returned to the area after the bombing for an investigation and a cleanup operation, which involved the removal of all bullet shells and cartridges.

The group said it interviewed a member of Afghanistan's National Police criminal investigations office who said his unit had searched around the site after the incident, but that "ISAF forces had collected all shells, magazines, cartridges from the spot and we could not find any trace or sign of them."

U.S. military officials were not available to comment on that allegation.

Afghan President Hamid Karzai has repeatedly pleaded for Western troops to show more restraint amid concern that civilian deaths shake domestic support for the foreign military involvement that he needs to prop up his government, increasingly under threat from a resurgent Taliban.

The initial U.S. military investigation concluded that the Marines' response was "out of proportion to the threat that was immediately there," the senior U.S. defense official said Wednesday in Washington.

The official spoke on condition of anonymity because the probe's results have not been released. The findings have been forwarded to U.S. Central Command, which has responsibility for U.S. military operations in the Middle East and Central Asia.

Another official, also speaking on condition of anonymity, said the initial military investigation concluded that there was a "reasonable suspicion" the Marines violated the rules for the use of deadly force, and that crimes, possibly including homicide, may have been committed in the aftermath of the convoy being struck.

One Marine was wounded in the blast, which also killed the bomber.

Army Maj. Gen. Francis H. Kearney III, head of Special Operations Command Central, opened an investigation into the incident after taking the highly unusual step of ordering the unit of about 120 Marines out of Afghanistan.

"We deeply regret the loss of life and casualties that resulted from the (suicide car bombing) and the actions that followed," Lt. Col. Lou Leto, spokesman at Kearney's command headquarters in Tampa, Fla., said in a statement. "We will work to prevent similar events from occurring in the future."

The Marines are in a special operations unit that deployed from Camp LeJeune, N.C., in January with the 26th Marine Expeditionary Unit. After Kearney ordered them out of Afghanistan, they returned to their unit's ships in the Persian Gulf.

The unit is one of four Marine Special Operations Command companies established since the command was created in February 2006. The one ordered out of Afghanistan was the first to deploy abroad.