Oromia Police Commissioner Ararsa Merdasa confirmed the attack on ethnic Amahras in Western Wollega, saying that out of the 50 were gathered by the suspected Oromo Liberation Army (OLA) rebels 28 of them were killed.
He said the attack also left 12 other civilians injured.
Three suspected OLA rebels were killed in a counteroffensive by security forces in the aftermath of the killings.