Armed men killed 13 recovering drug addicts at a Mexican rehab center near the border city of Tijuana in an attack officials blamed on the country's warring drug gangs.
The brutal killings late Sunday followed the slaying of 14 youths at a party in another city on the US border, Ciudad Juarez, at the start of the weekend, amid a wave of drug violence that has left more than 28,000 dead since 2006.
"There were 13 people killed. They were all recovering drug addicts," said a police official in Tijuana, declining to be named.
According to a witness, gunmen lined up their victims against a wall and executed them.
As many as 134 tonnes of marijuana were seized in Tijuana last week in the largest drug bust in Mexican history, and some law enforcement officials suggested Sunday's killings might have been connected to this event.
The rehabilitation center is located in the same neighborhood where the drugs were seized and destroyed.
The seizure came as, across the border, Californians were due to vote on November 2 in a referendum over whether to legalize marijuana for recreational use.
Mexico's border regions have witnessed much of the drug conflict over lucrative trafficking routes into the United States, with relentless violence in and around Ciudad Juarez.
The Tijuana attack came as officials claimed that violence was abating there.
It followed the weekend killing of 14 people in Juarez, in which gunmen stormed a home and shot indiscriminately at partygoers, including many teenagers.
That attack, which began at pre-dawn hours on Saturday, was the third such massacre this year in Mexico's most violent city, and it bore the chilling hallmarks of a drug cartel attack.
The young gunmen screamed insults as they opened fired on the crowd, according to witnesses who told the police the hail of bullets lasted five minutes.
Also in Ciudad Juarez, three police officers were killed outside their patrol vehicle on Sunday, when police in the southwestern state of Guerrero found the executed bodies of six men outside the resort city of Acapulco, local officials said.
Three more bodies were found with their hands tied in a northern suburb of Acapulco on Monday.
Drug violence has claimed more than 7,000 lives nationwide in 2010, making it the deadliest year since President Felipe Calderon launched a war on drug cartels in 2006, involving more than 50,000 troops.