Most of those who died were teenagers or young adults
The mayor of the Mexican border city of Ciudad Juarez says he believes a gun attack on partying high school students that left 16 people dead was random.
Mayor Jose Reyes Ferriz said none of the victims had criminal records and were "good kids, students, athletes".
More than a dozen gunmen opened fire on a house where the young people had gathered at the weekend.
President Felipe Calderon said security strategies would be broadened in Ciudad Juarez, Mexico's most violent city.
Ciudad Juarez, which lies on the US border, is the scene of a vicious ongoing turf war between rival drug cartels and last year saw more than 2,600 murders.
Deploring the killings at the weekend, Mr Calderon said the violence there went beyond questions of organised crime and the response of the police.
"It is a complex problem that is not only criminal, but social," Mr Calderon said, adding that his government would be drawing up a much broader and more integrated security strategy for the city in the coming days.
Ciudad Juarez has had about 10,000 troops and extra police officers patrolling its streets over the past year.
Last month, amid accusations of increased human rights abuses by soldiers, the government said it would scale down the military presence and boost police numbers by 2,000.
The weekend's shootings shocked residents of a city that has seen countless deaths over the past few years.
"There is no logical explanation," Mr Reyes told Mexico's MVS Radio as authorities announced a reward of 1m pesos ($76,200) for information leading to the arrest of the attackers.
"This is something that worries us - gratuitous or random criminal acts. It goes way beyond what had been happening and puts Ciudad Juarez in even greater danger."
Witnesses said the gunmen drove up to the house in several cars late on Saturday or in the early hours of Sunday.
They began shooting at people from outside the property before moving inside, and pursued some of the youngsters trying to escape over a fence.
The number of dead rose from 14 to 16 on Monday, as two more victims died in hospital. Several people were injured but none are believed to be in a critical condition.
"Who did it? Why such hate?" said Mari Cruz, whose son and nephew died in the attack.
"They were just boys who were not in any gangs."