CIUDAD JUAREZ, Mexico – At least 30 gunmen burst into a drug rehabilitation center in a Mexican border state capital and opened fire, killing 19 people and wounding four, police said. Gunmen also killed 20 people in another drug-plagued northern city.

The killings marked one of the bloodiest weeks ever in Mexico and came just weeks after authorities discovered 55 bodies in an abandoned silver mine, presumably victims of the country’s drug violence.

The bullet-riddled bodies of 18 men and two women were found Friday in five different parts of Ciudad Madero, a city in the northeastern state of Tamaulipas, where violence has surged this year amid a turf battle between the Gulf cartel and its former ally, the Zetas gang.

Police had no information on suspects.

It was the deadliest day in Tamaulipas drug violence since 18 gunmen died in clashes with soldiers in April.

Another round of killings occurred late Thursday at the Faith and Life drug rehab center in Chihuahua city, about 210 miles south of Ciudad Juarez and the border with El Paso, Texas, state police spokesman Fidel Banuelos said.

More than 60 people have died in mass shootings at rehab clinics in less than two years. Police have said two of Mexico’s six major drug cartels are exploiting the centers to recruit hit men and drug smugglers, often threatening to kill those who don’t cooperate. Others are killed for failing to pay for drugs or betraying a dealer.

The men at the Faith and Life center were shot after they were roused out of bed and placed face-down along a hallway, the center’s director, Cristian Rey Ramirez, told The Associated Press.