Gunmen lined up six young men and killed them in a neighborhood in Monterrey, the capital of the northern Mexican state of Nuevo Leon, officials said.

"A group of armed men arrived in a taxi and an SUV in one of this city's neighborhoods and killed them like a firing squad with large-caliber weapons in front of a house," a state Security Council spokesman told Efe.

The gunmen caught the victims by surprise Wednesday at the intersection of Planicie and Los Rodriguez streets, the state Security Council spokesman said.

Investigators gathered up numerous bullet casings from AK-47 assault rifles at the crime scene.

The victims were known drug dealers in the neighborhood, residents said.

The killings occurred about 24 hours after gunmen killed three men and a woman, and wounded a second woman at a house in a city outside Monterrey.

The gunmen arrived in the Las Sabinas neighborhood of Guadalupe, a city in the Monterrey metropolitan area, in two compact cars, a spokesman for the Nuevo Leon State Investigations Agency, or AEI, said.

The gunmen opened fire with AR-15 and AK-47 assault rifles on five people who were talking outside a house on Monte Alban street, the AEI spokesman said.

Monterrey, Mexico's most important industrial city, and its suburbs have been battered by a wave of drug-related violence since March 2010, when three rival cartels reportedly went to war with Los Zetas, considered the country's most violent criminal organization.

Los Zetas has been battling an alliance of the Gulf, Sinaloa and La Familia drug cartels, known as the Nueva Federacion, for control of the Monterrey metropolitan area and smuggling routes into the United States.

Heriberto Lazcano Lazcano, known as "El Lazca," deserted from the Mexican army in 1999 and formed Los Zetas with three other soldiers, all members of an elite special operations unit, becoming the armed wing of the Gulf drug cartel.

After several years on the payroll of the Gulf cartel, Los Zetas, considered Mexico's most violent criminal organization, went into the drug business on their own account and now control several lucrative territories.

Los Zetas has been blamed for several massacres in recent years.

The cartel was accused of being behind the Aug. 23, 2010, massacre of 72 migrants, the majority of them from Latin America, at a ranch outside San Fernando, a city in the northeastern state of Tamaulipas.

Los Zetas has also been blamed for the massacre of 27 peasants in May at a ranch in Guatemala's Peten province, which borders Mexico and Belize.

Zetas gunmen set fire to the Casino Royale in Monterrey, the capital of Nuevo Leon, on Aug. 25, killing 52 gamblers and employees trapped inside, most of whom died of smoke inhalation.

A total of 267 murders were registered in the industrial city in 2009, with the figure rising to 828 in 2010 and nearly 2,000 so far this year, according to official figures.