A client at a drug rehab center in the Mexican border city of Tijuana said Monday that a gang of armed men burst into the building and gunned down 13 recovering addicts there.
Prosecutors have not yet confirmed the number of dead. Police at the scene say at least 10 were killed.
The witness, who asked to be identified only by his first name, Jesus, for fear of reprisals, said he was attending a movie showing on the first floor of the center, and had stepped out for something to eat when the attack 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. Other clients sleeping upstairs in the center also survived. There are normally about 45 clients at the center.
The attack on the ramshackle, privately run center is the first such mass killing at a rehab center in Tijuana, a city praised by some for its anti-gang efforts.
Several such attacks have killed dozens of recovering addicts in another border city, Ciudad Juarez, and a voice was heard over a police radio frequency later saying "this is a taste of Juarez."
While police have not identified the motive in the Tijuana slayings, drug gangs have attacked such centers before to target rival gang members.
In Ciudad Juarez, prosecutors' spokesman Arturo Sandoval said three municipal police officers were found shot to death outside their patrol vehicle on Sunday.
And in the southern Pacific coast state of Guerrero on Sunday, state police found the bound, executed bodies of six men on a highway outside the resort city of Acapulco.
The men had been blindfolded, their hands and feet bound, and shot to death with assault rifles, the state Public Safety Department reported.
The killers left three handwritten messages with bodies, a tactic frequently employed by Mexico's drug gangs to threaten their rivals or authorities, but police routinely do not reveal the contents of such messages.
Nationwide, more than 28,000 people have been killed in drug gang violence since December 2006, when President Felipe Calderon deployed soldiers to battle the cartels in their strongholds in northern Mexico and along the Pacific coast.
While the government says most of the dead were involved in the drug trade, innocent bystanders have also died, like three people killed in the crossfire of a shootout between gunmen, police and soldiers in northern Coahuila state Sunday.
The victims were a 14-year-old boy and two women aged 18 and 47, according to a statement by the state prosecutors' office.
The statement said gunmen traveling in two vehicles opened fire on a convoy of federal police officers and soldiers in the city of Saltillo, Coahuila. The officers and soldiers returned fire.
It was not clear who fired the shots that killed the bystanders, but the state attorney general's office said it was investigating and expressed condolences to the victims' families.
Birthday party massacre
"They are civilians who unfortunately died in the exchange of gunfire," it said, describing a running series of confrontations between police and assailants who allegedly fired shots into the air to clear bystanders from their path at one point.
In Ciudad Juarez, meanwhile, the death toll from a birthday party massacre late Friday rose to 14 when an 18-year-old man died of his wounds.
Nineteen people were wounded in the attack on two private homes where about four dozen partygoers had gathered for a teenager's birthday.
The dead identified so far were 13 to 32 years old, and the majority of the victims were high school students, a survivor said.
While investigators said they have not yet identified the perpetrators or a motive, police found 70 bullet casings from assault weapons typically used by drug gangs at the scene of the shootings. Cartel violence has killed more than 2,000 people so far this year in the city, which is across from El Paso, Texas.
Drug gangs have increasingly attacked private parties they believe members of rival gangs might be attending; innocent partygoers are often killed in such attacks.
On Sunday, prosecutors in northern Chihuahua state, where Ciudad Juarez is located, said they were searching for a man known only by his nickname, "The Mouse," who was apparently the target of the gunmen.
The man was reportedly wounded in the Friday shooting, but has disappeared. Investigators said they believe he can provide information on who was trying to kill him.
Memorial services were held Sunday for some of the victims of Friday's attack, and prosecutors said guards had been provided to protect the services.