Two men ambushed a backyard cookout near Pittsburgh, killing a pregnant woman, her 8-month-old fetus and four other adults, because they wanted retribution for the 2013 slaying of a friend, authorities said Thursday. Robert Thomas, 27, and Cheron Shelton, 29, both face five counts of criminal homicide, one count of homicide of an unborn child, three counts of aggravated assault, three more of aggravated assault with a deadly weapon, six counts of reckless endangerment and one count of criminal conspiracy to commit murder.