Armed men burst into a drug rehab center in Tijuana, killing 13 people.
A witness, who asked to be identified only by his first name, Jesus, for fear of reprisals, said he had stepped out for something to eat when the attacked occurred late Sunday.
When he returned, his fellow clients told him the attackers made the addicts lie on the floor, and then sprayed them with bullets, killing 13. Other clients sleeping upstairs in the center also survived. There are normally about 45 clients at the center.
Prosecutors had not yet confirmed the number of dead. Police at the scene said at least 10 were killed.
It was the second massacre of the weekend in Mexico: 14 people were killed Friday night when gunmen stormed a birthday party in another border city, Ciudad Juarez.
The attack on the ramshackle, privately run center in Tijuana is the first such mass killing at a rehab center in the city, praised by some for its anti-gang efforts.
While police have not identified the motive in the Tijuana slayings, drug gangs have attacked such centers before to target rival gang members.
