Mexico's governmental National Human Rights Commission says police in the southern state of Guerrero probably fired the shots that killed two demonstrators at a violent protest last month.
Commission President Raul Plascencia says investigators haven't proved who fired the shots, federal or state police.
He says police failed to help wounded demonstrators or preserve evidence after the Dec. 12 incident.
Plascencia said Monday it is unclear who set fire to a gasoline station on the highway that the protesters were blocking. A station employee died later of burns.
Students from a rural teachers college blocked the road to demand more funds for the school and battled police trying to clear the highway.