The 42-year-old man, identified as Dilawar, has been detained in Estremera prison, a suburb of Madrid, since he confessed in January to being involved in a triple homicide. He is now suspected of killing his cellmate on Thursday, the Madrid Region Superior Court of Justice said.
According to media reports, the victim was beaten and found dead in the cell he shared with Dilawar. Dilawar had in January surrendered to police and admitted his involvement in the triple murders in December of two sisters and their disabled brother.