US soldiers found mass graves north of Baghdad next to a torture center where chains were attached to blood-spattered walls and a metal bed frame was still connected to an electrical shock system, the military said Thursday. Separately, at least 13 Iraqis were killed when a suicide bomber targeted a group of people who had gathered around US soldiers handing out holiday gifts, local authorities said. It was not immediately known if any soldiers were killed or wounded. In the capital, a car bomb exploded outside a liquor store in central Baghdad, killing three civilians and wounding another nine, police said. The torture center, which the military said it thinks was run by al-Qaida in Iraq, was found based on tips from local Iraqis. Graves containing 26 bodies were found nearby.