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Gunmen opened fire on people gathered at a street corner in the northern city of Monterrey, killing two adults and a 13-year-old boy, authorities said Tuesday.

The boy's twin brother and two men were wounded in the attack Monday night, the Nuevo Leon state attorney general's office said.

Authorities had no immediate suspects. While the motive was unclear, Mexican drug cartels have recruiting younger and younger members.

The government has increasingly detained youths under 18 for drug-related crimes. Last month, authorities arrested a 14-year-old boy who they alleged worked as an assassin for a drug gang in central Mexico.

Monterrey, Mexico's third-largest city, has been besieged by fighting between the Gulf cartel and the Zetas drug gang.

In Apodaca, another town in Nuevo Leon state, a shooting broke out Monday night between marines and suspected cartel gunmen.

One marine and one suspect were injured, though not seriously, the Mexican navy said in a statement. Five suspected members of the Zetas were arrested, including a minor, it said.

A spokeswoman for the town government said electricity and telephone service went down for three hours during the shootout, though the cause was unclear. Navy officials said they had no information on that.

Gunmen used vehicles to block five roads in Apodaca — a common cartel tactic to impede movement of government security forces — according to the town spokeswoman, who spoke on condition of anonymity.

In the border city of Ciudad Juarez, across from El Paso, Texas, federal police announced they had arrested three U.S. citizens in a sport utility vehicle loaded with 159 packages of marijuana hidden in the bodywork and gas tank.

Police said the men were acting suspiciously, were followed by police and gave contradictory answers when stopped for questioning. An inspection of the vehicle with gamma rays uncovered the packages, which weighed a total of 173 pounds (78.7 kilograms), police said.

The vehicle had New Mexico license plates, but the U.S. Embassy could not immediately confirm the nationality of the detainees.