Who would murder one of the most respected priests of South America?

Óscar Romero, a respected Catholic priest, called on soldiers, as Christians, to put down their arms and stop carrying out the governments order to strip citizens of the most basic human rights...for this he was assassinated. 

For over 30 years, his murder has gone unsolved.

Who would murder a priest who only wanted to stop the injustice? And more importantly, why is it that, with substantial evidence naming the murderers involved, was nothing done to convict those guilty of murdering the country's beloved archbishop?