The fifteen dead youths went to high school at CBTIS 128. CBTIS stands for Center for Industrial Technology and Services Secondary Education. CBTIS schools are spread throughout Northern Mexico, and each one is numbered. American football is popular in Northern Mexico and CBTIS schools have a football league; students in the fourth to sixth semester, according to a story in Norte by Beatriz Corral Iglesias (click here) are eligible to play in the Double A category, Liga de Futbol Americano AA.
For weeks students on the Double A team planned a birthday party for one of their fellow athletes. The word went out that the Double A's were having a party. Everyone in Villas de Salvarcar knew about it.
But there is another group calling itself the Double A's (for "Artist Assassins") in Juarez: a violent street gang said to be involved in drug trafficking for Chapo Guzman, head of the Sinaloa Cartel. Many homicides in Juarez are said to be the product of rivalries between the Double A's and the Mixcles, on the one hand, and the Aztecas or the La Linea gangs, associated with the Juarez Cartel, on the other.
So apparently when La Linea gang members found out the Double A's were having a party, they confused the high school football team with the rival gang and decided to interrupt the party.