Gunmen in the Mexican city of Tijuana have shot dead 13 people at a drug rehabilitation centre, say police.
The attack appeared to be connected to the long-running violent conflict between drugs gangs in the the country, in which thousands have been killed.
On Saturday, 14 people, most of them teenagers, were shot dead at a party in the border city of Ciudad Juarez.
Last week, police in Tijuana destroyed 134 tonnes of cannabis - the largest drugs haul ever seized in the country.
The drugs, with an estimated street value of at least $340m (£214m), had been wrapped in 15,000 separate packages.
Police in Tijuana, just over the border from San Diego, California, said the latest killings happened when an armed gang burst into the Camino drug treatment centre in the city.
They lined the victims up and shot them with high-powered weapons, El Universal newspaper reported.
Drugs rehabilitation centres have been attacked by gunmen before - observers say the gangs accuse the clinics of protecting rival dealers.
Police also believe drug cartels use the clinics to recruit hitmen and smugglers, threatening to kill those who fail to co-operate.
The Tijuana deaths came as families mourned the 14 people shot dead on Friday night in Ciudad Juarez.
At least 10 people were also injured as the gunmen burst into the party, police said.
"The victims were in the backyard of the house having a party when hooded men, in dark uniforms and with rifles, arrived in several vans, broke in and began shooting indiscriminately at those inside," a police official told the AFP news agency.
Police said the victims were aged 14-20. There have been no reports of any arrests.
"This can't be happening. Today it's them who are killed, and tomorrow who will be next?" the sister of one of the victims told Reuters, as she attended his wake.
The attack was similar to one earlier this month, in which six people were killed at a party.
The Mexican police and army are struggling to control armed cartels operating in cities including Tijuana and Ciudad Juarez.
The Sinaloa and Carillo Fuentes Organisation (also known as the Juarez cartel) gangs are also competing in Ciudad Juarez over lucrative drug smuggling routes into the US, making the city one of the bloodiest front lines in Mexico's drugs war, despite the presence of some 4,500 police and soldiers.
More than 28,000 people have died in the drugs war since President Felipe Calderon ordered the army into the fight in 2006.
The policy has had some successes arresting drug lords, but that has not led to a decline in the number of killings, or the level of kidnappings, extortion and human trafficking that the gangs also engage in.