The death toll of the weekend massacre in Ciudad Juarez, one of the deadliest cities in the world, rose to 16 after three more victims died at hospitals, according to the Chihuahua state attorney general's office.
The young people were gathered to watch a boxing match Saturday night when two trucks drove up loaded with armed men who opened fire. Some people were shot as they tried to flee and their bodies were found near neighboring homes.
The victims' ages ranged from 15 to 20.
Ciudad Juarez Mayor Jose Reyes Ferriz said police were still trying to determine the motive and had not ruled out that any of the victims were involved with drugs. However, he said they seemed to be "good kids, students, athletes."
Witnesses and relatives insisted the young people were targeted by mistake or randomly. If that's the case, Reyes said it would be unprecedented even for the violent Ciudad Juarez, where rampant drug warfare is common.
"There is no logical explanation, a concrete reason for this event. This is something that worries us, gratuitous or random criminal acts," Reyes told MVS Radio. "It goes way beyond what had been happening and puts Ciudad Juarez in even greater danger."
Authorities have offered a reward of 1 million pesos ($76,200) for information leading to the capture of the gunmen. Reyes said police have received five calls with tips that could be useful, but he did not elaborate.
"The people of Juarez must demand that this is investigated thoroughly and we must demand justice for such deplorable crimes," he said at a news conference later Monday.
A large crowd gathered at the scene of the attack in Villas de Salvarcar, a working class neighborhood of cinderblock homes partly surrounded by a fence topped by barbed wire. Some people had set down candles.
One woman, who identified herself only as Martha out of fear for her safety, said both of her sons died in the attack. She said they both played on their high school football team.
"Who took my two boys?" she said. "What harm could they have done to anyone?"
A girl who gave only her first name, Linda, said her cousin, 17-year-old Jose Adrian Encina, was killed. She described him as a good student who aspired to be a doctor and had received an academic recognition award from Chihuahua Gov. Jose Reyes Baez.
In another attack in Ciudad Juarez on Monday, armed men burst into a bar around dawn and killed four men and a woman, said Arturo Sandoval, a spokesman for the regional prosecutors' office.
Ciudad Juarez, a city of 1.3 million people which faces El Paso, Texas, is home to several drug cartel bosses who are battling for turf.
More than 2,600 people were killed last year alone. Authorities say most of the dead have ties to drug gangs but civilians have been caught in the cross fire in increasing numbers. Last year, among the dead were university professors and an honor student.
Elsewhere, gunmen killed 10 people and wounded 15 in a bar in Torreon, a city in the northern state of Coahuila.
Drug violence has surged in many parts of Mexico since President Felipe Calderon deployed thousands of troops in 2006 to crush powerful cartels. More than 15,000 people have been killed in gang violence since.