Mexican police say gunmen shot 13 recovering drug addicts inside a rehabilitation center, killing at least 10 of them.
Witnesses say the armed men burst into the center, lined up the clients and opened fire on them.
The attack took place Sunday in the city of Tijuana, near the U.S. border.
Police did not immediately identify a motive. But authorities suspect the killings may be linked to drug gangs.
Mexican security forces have been engaged in a brutal struggle against the country's violent drug cartels since President Felipe Calderon took office in 2006.
Nearly 30,000 people have been killed since the campaign began.
Last week, Mexican security forces seized 105 tons of marijuana in Tijuana, the largest Mexican drug bust in recent years.
Some information for this report was provided by AP and AFP.