Afghanistan ? An Afghan human rights group asserted Saturday that U.S. Marines acted illegally last month when they fired on scores of civilian vehicles on a busy highway after their convoy was attacked with a car bomb.

Twelve Afghan civilians were killed and dozens of others injured in the March 4 shootings outside the eastern city of Jalalabad.

The incident sparked outrage among Afghans and put new pressure on the government of President Hamid Karzai, which repeatedly has appealed to Western forces to be more mindful of civilian safety.

The report by the Afghanistan Independent Human Rights Commission follows a weeks-long investigation by the group, which says it interviewed about 50 witnesses, local authorities and hospital officials. The findings come as the U.S. military weighs whether criminal charges should be brought against the Marines involved.