Story highlights Jose Armando Moreno Leos, 13, last month had admitted being a hit man, authorities say

He was found this week murdered execution-style, a state's attorney says

He had been released from custody in compliance with Mexico's constitution

A 13-year-old boy who had confessed to being an assassin for a Mexican drug cartel was among six people found murdered execution-style, authorities in the central Mexican state of Zacatecas have confirmed.

The boy, identified as Jose Armando Moreno Leos by officials, was arrested only three weeks ago by the Mexican Federal Police, according to Arturo Nahle Garcia, state's attorney in Zacatecas.

"After being detained, he confessed to authorities that he had participated in at least 10 homicides and that he was somebody who was good at shooting with a high-caliber weapon," Nahle Garcia said.

After his February arrest, the Federal Police released the boy into the custody of the Mexican Attorney General's Office, which later set him free in compliance with the law. The Mexican constitution prohibits the incarceration of anybody under the age of 14. The constitutional ban also applies to correctional facilities.

JUST WATCHED Are armed forces behind disappearances? Replay More Videos ... MUST WATCH Are armed forces behind disappearances? 02:49

JUST WATCHED Community police take on Mexican cartels Replay More Videos ... MUST WATCH Community police take on Mexican cartels 02:30

JUST WATCHED Smugglers shoot marijuana across border Replay More Videos ... MUST WATCH Smugglers shoot marijuana across border 02:31

According to Nahle Garcia, Moreno Leos missed a court appearance on February 20. "Only his mother appeared before the judge to say that she had lost control of her son after he left the house at about age 11," the prosecutor, said.

The court appearance was to determine custody and measures to help the teenager leave behind a life of crime.

The boy's body was found Thursday alongside a highway in the municipality of Morelos.

The bodies of five other people, four females and one male, were also found at the same location. Officials say they had all been shot execution-style with high-caliber weapons.

"They all appeared to be young people, but we're still in the process of positively identifying the bodies," Nahle Garcia said.

This is not the first time a teenager has admitted being a hit man for organized crime in Mexico. In 2011, authorities arrested a 14-year-old boy, identified only as "El Ponchis" -- "The Cloak" -- who admitted on camera that he had brutally killed people. In a video obtained by CNN, he told a military interrogator that he had beheaded four people.

El Ponchis was found guilty and sentenced to three years in a correctional facility, the maximum allowed under Mexican law.

Speaking about the most recent incident, Nahle Garcia said he's not surprised. "It's really unfortunate, but we're seeing more and more young men who drop out of school and end up selling drugs on the streets," he said. "They all end up the same. They either end up in jail or the cemetery."