Attackers in two vehicles pulled up to the houses in a lower-middle-class Ciudad Juarez neighborhood late Friday and opened fire on about four dozen partygoers gathered for a 15-year-old boy's birthday party.
The dead identified so far were 13 to 32 years old, including six women and girls, Chihuahua state Attorney General Carlos Salas told reporters at a news conference at the crime scene. The majority of the victims were high school students, a survivor said.
Relatives of the victims gathered outside prosecutors' office, some weeping laments, some shouting demands for justice. All asked not to be identified for fear of reprisals.
"Why, my God, why," shouted the mother of Daniel Figueroa, 16, after she identified her son's body.
"I want to be dead, with my daughter," the mother of a 19-year-old woman killed in the attack told a reporter. "This cannot go on. We want justice, even though nothing can bring her back."
Salas said a total of 20 people were wounded, including a 9-year-old boy. Authorities earlier gave lower numbers for the wounded because some victims were taken by relatives to hospitals throughout the city and were not immediately located.
Residents of Ciudad Juarez, one of the world's deadliest cities, no longer go out much to celebrate because of a violent turf war between the Sinaloa and Juarez cartels, who frequently attack their rivals in bars, restaurants, drug rehab centers and other public places.
One survivor of Friday's massacre said the birthday boy's mother had decided to hold the party at their home, precisely because she thought it was safer. The party spilled over into the neighboring home.
The 16-year-old boy, who did not want to be identified, said an attacker entered one of the homes and asked partygoers about a car parked in front of the home, suggesting the killers may have been following the vehicle. He survived the attack by throwing himself to the floor and other partygoers fell on top of him, shielding him from the bullets.
The survivor said the gunman he saw appeared to be about 20, wearing a baseball cap and carrying a pistol, and simply opened fire after no one answered his questions.
Police found 70 bullet casings from assault weapons typically used by drug gangs whose bloody turf battles have killed more than 2,000 people this year in Ciudad Juarez, across the border from El Paso, Texas.
Salas said the attackers escaped, and police said had no immediate information on any suspects or possible motive. Salas expressed outrage at the situation.
"The criminals do not respect families or their homes," Salas said. "How can they wound young women, how can they severely wound a 9-year-old boy, how is it possible that a group of youths gathering to hold a birthday party wind up like this?"
The Interior Department condemned the killings in a statement and pledged "to help the efforts of state and local authorities re-establish order in Ciudad Juarez."
Some recent attacks on private homes have resulted in apparently innocent people being killed, either because a targeted person was at the gathering or because gunmen simply had the wrong address.
Most recently, attackers stormed two homes on Oct. 17, killing seven people at a party and two others in another house nearby.
And in January, gunmen massacred 15 people at a party in a house not far from the site of Friday's killings. Most of the victims were teenagers, students and athletes.
Investigators later said the attack was apparently carried out by Juarez cartel gunmen looking to kill allies of the Sinaloa cartel. There is no evidence the youths were the targets, and police said the killers may have hit the wrong house.
The city was outraged by the January massacre, leading President Felipe Calderon's government to vow to implement a new strategy for restoring order in Ciudad Juarez, where the army had by then had replaced the disorganized, outgunned local police.
In April, federal police took over public security duties from the army, and about 5,000 federal officers were deployed in Ciudad Juarez.
The federal government also stepped up social programs to try to break the cycle of poverty, broken homes and lack of opportunities that make the city's youths a fertile recruiting ground for the gangs.
Cash aid programs, neighborhood improvement initiatives, educational and job-training programs were part of the new strategy, together with ubiquitous convoys of blue federal police trucks patrolling "safe corridors" throughout the city.
But in light of the recent mass attacks, it is unclear whether the new strategy for the city is having an effect so far. While the bustling industrial hub was known mainly throughout the 1990s for the grisly series of murders of more than 100 young women, the city's youths now bear the brunt of the violence.
In an interview with The Associated Press earlier this month, President Felipe Calderon said the Juarez strategy is a long-term policy.
"We cannot think that all the ground lost regarding opportunities for these young people can be recovered in a few weeks," Calderon said. "If we are building five new high schools and two universities, don't tell me it's not working if classes started a month ago."