Two inmates died and at least seven more suffered knife wounds about 9 a.m. Monday when a riot broke out at the La Reforma prison in San Rafael de Alajuela.
Investigators suspect that the disorder was a cover for the assassination of two inmates, identified by the names of William Friny and Alberto Lowe.
The Judicial Investigating Organization said that there were about 800 inmates taking the sun in the patio of the 1B wing of the prison when the disorder took place.
The riot appears to have been sparked by an argument that grew to include many others.
The two murdered men were elsewhere in their cell, and it appears that murderers sought them out. They were dead on arrival at the Hospital San Rafael de Alajuela. Two other inmates were hospitalized. Others were treated at the prison.
The Policía Penitenciaria was able to control the situation with the help of the Fuerza Pública, but two men had been stabbed multiple times by then.