An official in South Sudan says 57 people have been killed and 40 are missing after a tribal revenge attack in the wake of a massive assault last month that sent tens of thousands of people fleeing. County Commissioner Simon Hoth said Friday that a group of 400 armed attackers from the Murle ethnic group set fire to three villages and killed 57 members of the Lou Nuer on Wednesday. Hoth says 40 people are missing. The attack happened in Jonglei state, site of a massive tribal attack by the Lou Nuer against the Murle starting on Dec. 23 and lasting several days. Tens of thousands fled, and hundreds are feared dead. South Sudan became the world's newest country last year after decades of war with Sudan, but it is now experiencing massive internal violence.