The U.N.'s human rights office on Tuesday added grim new details about the massacre of more than 100 people in a rural area in Syria last week, saying that most of the dead were shot at close range, some of them women and children killed in their homes. The brutality of the Houla killings, documented in gruesome amateur videos of scores of bodies laid out before burial, sparked widespread international outrage and raised new questions about the ability of an international plan to end 15-months of violence in Syria. The U.N. said that at least 108 people, including 34 women and 49 children, were killed in an attack that began on Friday and continued through the night on Houla, a group of poor farming villages northwest of the central Syrian city of Homs. On Tuesday, spokesman Rupert Colville of the U.N. High Commissioner for Human Rights said U.N. monitors who visited the area found that fewer than 20 of the dead were killed by artillery fire. The rest appeared to have been shot at close range. "What is very clear is this was an absolutely abominable event that took place in Houla, and at least a substantial part of it was summary executions of civilians, women and children," he told reporters in Geneva. At this point, it looks like entire families were shot in their houses. Colville said information from U.N. observers and other sources indicated that many of the victims were killed in the village of Taldaw in two separate incidents. Locals blamed the killings on pro-regime militias known as shabiha, which sometimes act "in concert" with government forces, he said. He said a fuller investigation was needed before he could comment on that, and called on Syria to allow free access to U.N. investigators. The killings have emerged as a potential turning point in Syria's conflict, prompting the strongest condemnation yet from Russia, a staunch Syrian ally. Annan is trying to salvage a peace plan for Syria based on a cease-fire that was supposed to begin on April 12 but has never really taken hold, casting doubt on the rest of the plan. Anti-regime activist posted videos online of explosions in the villages of Houla, dismembered bodies lying in the streets, then row upon row of dead laid out before being buried in a mass grave. Some of the videos showed dozens of dead children, some with gaping wounds. Houla activists reached by Skype said government troops shelled the area after anti-government protests on Friday and clashes with local rebels. Later, shabiha from nearby villages swept through the area, stabbing residents and shooting them at close range. Assad's regime has denied any role in the massacre, blaming the killings on "armed terrorists" who attacked army positions in the area and slaughtered innocent civilians. But it has provided no evidence to support its narrative nor has it given a death toll. U.N. observers said they found tank and artillery shells in Houla after the attack, but stopped short of blaming regime forces for Friday's killings.