A video provided by Hamourabi Human rights group shows bodies being loaded onto a truck, which Hamourabi says belong to a family of 15 shot dead in their home in Haditha, in western Anbar province, March 23, 2006.

    The investigation is linked to two incidents. One took place on March 15 in a village north of Baghdad, where 11 civilians, including five women and four children, were killed as U.S. troops raided a house to search for an Al-Qaida suspect.

    In the other incident, U.S. marines allegedly shot 15 civilians dead on purpose near the western Iraqi town of Haditha last year.

    The killings are part of a series of incidents in which U.S. forces have repeatedly been accused of using excessive force in counter-insurgency operations in Iraq.

    International and U.S. human rights organizations have been urging the U.S. military to take "all feasible precautions" to minimize civilian casualties.

    In Iraq, civilians often falls victim to U.S. soldiers, who routinely open fire on approaching vehicles in fear of suicide bombers.

    The most publicized such incident took place one year ago when U.S. troops shot dead an Italian secret service agent at a checkpoint as he escorted a just-released hostage to Baghdad airport. Enditem