At least 12 people have been seriously injured in a clash between rival gangs at a youth detention centre in Guatemala, police say.
The attack was said to be in retaliation for the August killings
The violence started when members of the notorious Mara 18 gang threw grenades at rivals from the Mara Salvatrucha, officials said.
The same gangs were involved in riots which left 35 inmates dead last month.
The gangs, or maras, emerged in Los Angeles and spread to Central America when gang members were deported.
Officials say the latest attack may have been in retaliation for the August fighting, in which many members of the Mara 18 were killed.
After the killings, the Guatemalan authorities promised to impose stricter controls in jails, but also acknowledged that the prison system was close to collapse.
One of the wounded Mara Salvatrucha members told journalists that they had asked to be kept apart from the other gang because "we knew they were going to attack us".
The maras were formed in the early 1980s by Salvadorean immigrants who had fled the civil war and settled in Los Angeles.
They were later joined by Hondurans, Guatemalans and other migrants. Many of them were deported back to their countries when Central America's civil wars ended.
Now Honduras, El Salvador and Guatemala are struggling to contain the activities of the maras, which are blamed for a wave of killings and robberies across the region.