CIUDAD JUAREZ -- An armed gang killed nine people during mass in a drug rehabilitation center in this northern Mexican border town, and eight others died in separate attacks in the area overnight, police said Thursday.

The gunmen "entered (the building) shooting at people who were praying" late Wednesday, police said, adding that 10 more people were wounded.

After the shooting, the assassins left the scene and calmly passed a group of security forces, who did nothing to detain them, said a statement from the municipal office of public security, quoting witnesses.

In an escalation of drug-related violence in Mexico's northern border regions, two people were also killed in a nearby rehabilitation center last weekend.

Meanwhile, five men were kidnapped and later executed in Chihuahua town, capital of Chihuahua state, and three others were violently killed in separate attacks in Ciudad Juarez.

Federal authorities have deployed more than 36,000 soldiers across the country, including 2,500 in Ciudad Juarez, in an effort to combat drug trafficking and related violence, but some 2,000 people have been killed so far this year.

Authorities attribute some 780 assassinations in Ciudad Juarez this year to turf wars between drug cartels.