Three inmates were murdered in a prison in Chalatenango, in the north of El Salvador, while a fourth inmate is missing, in another example of the crisis engulfing prisons across the region.
The bodies of the dead prisoners were found on Saturday, each in different sections of the prison.
Earlier the same day, the entrance to a tunnel was found in the bathrooms of another prison in San Francisco Gotera, in Morazan. It was 18 meters long, and had a diameter of 80 cm, running 2 meters below the surface, and its entrance was concealed by a toilet.
InSight Crime Analysis
The deaths are another sign of the disorder and violence of El Salvador's prison system. Murders of inmates are common, with four killed in Quezaltepeque prison in San Salvador in November 2011.
Prison director Nelson Rauda said that the latest deaths were likely related to struggles between prisoners for control over territory within the facility.
Earlier this month a United Nations human rights panel expressed concern at the state of the country's prisons, highlighting the "extreme overcrowding" and the problem of arbitrary detention. Similar problems have been brought to the world's attention by incidents across the region in recent weeks, with the massacre of 44 prisoners in Mexico, and a fire that killed over 350 inmates in Honduras.