Gunmen killed 13 young people and injured 20 others in the Mexican border town of Ciudad Juarez on Friday night.
The attackers arrived in two vehicles, stormed the homes where about four dozen neighbors were celebrating the birthday of a 15 year old.
Most of the 13 fatalities were high school students, one survivor said.
The violent turf war between the Sinaloa and Juarez cartels has made Ciudad Juarez – just across the border from El Paso, Texas – one of Mexico’s deadliest cities. More than 2,000 people have been killed this year.
Rival cartels often gun down people in public venues, including restaurants, bars and even drug rehab centers.
One 16-year-old eyewitness said the gunman he saw – who looked about 20 years old – opened fire when no one answered his questions.
Police found approximately 70 bullet casings from assault weapons usually used by gang members.
The mother of the birthday boy thought it would be safer to hold the party at home.
Just one week ago, gunmen killed seven people at a party and two others in house nearby.
AP contributed to this report.