(Adds Ukraine foreign minister, U.S.-backed investigation) By Marko Djurica and Abdelaziz Boumzar BUCHA, Ukraine (Reuters) – International outrage spread on Monday over civilian killings in northern Ukraine, where a mass grave and tied bodies of people shot at close range were found in a town taken back from Russian forces, as Moscow shifted the focus of the fighting elsewhere. The deaths in Bucha, outside Kyiv, are likely to galvanise the United States and Europe into additional sanctions against Moscow, possibly including some restrictions on the billions of dollars in energy that Europe sti…