A dozen young offenders were hacked to death after a clash between two gangs in a Guatemalan youth detention centre.
Two of the victims had their heads blown off after grenades were thrown at them during the fight in Guatemala City.
All of the dead were from a gang called Mara 18, their slayers from a rival outfit called Mara Salvatrucha.
Only 10 guards were on duty at the time of the onslaught which left at least eight other people injured.
Interior Minister Julio Anleu said: "What we have here is a commando-style raid by armed gang members who made their way into the centre and opened fire."
This most recent battle between the two gangs was sparked by an initial clash on August 15 at a prison in Guatemala.
The fight led to more incidents at six other facilities that left a total of 35 people dead.
On September 6, gang members attacked a group of rivals with grenades at a Guatemalan youth detention centre, injuring 12 people.
On the streets, the gangs fight each other for control of neighbourhoods where they extort and assault people, and deal drugs, the Sky News reports.