Authorities say Edgar Jimenez Lugo confessed to killing four people whose beheaded bodies were hung from a bridge in Cuernavaca, a central colonial city that is popular with Mexican and international tourists alike and is known for its yearlong springlike climate.
Jimenez has been charged with the homicides and other crimes, including arms possession and transporting cocaine, prosecutors from Morelos state, where Cuernavaca is located, said in a statement late Wednesday.
Because he was charged as a juvenile, he faces a maximum three years in prison.
Prosecutors identified the boy only by his first name, but his family gave his full name to journalists.
Jimenez was born in San Diego, California, but grew up in Mexico, raised by his grandmother. He and his sister were arrested in December as they tried to board a plane to Tijuana, where they purportedly planned to cross the border and reunite with their mother in San Diego.
The two siblings allegedly worked for Julio "El Negro" Padilla, a reputed drug trafficker who authorities say has been fighting for control of the drug trade in Morelos. Morelos was formerly under the control of the Beltran Leyva gang, which broke up after alleged leader Arturo Beltran Leyva was killed in a shootout with Mexican marines a year ago.
The battle among remnants of the gang has caused an unprecedented spike in violence in Morelos and in neighboring Guerrero state, where the resort city of Acapulco is located.
Stories of a hit boy, maybe as young as 12, spread after a YouTube video appeared in November with teens mugging for the camera next to corpses and guns. One boy on the video alleged that "El Ponchis" was his accomplice.
A relative has said Jimenez was nicknamed "Ponchi" by his family because he was a pudgy child.
When he was handed over to federal prosecutors, the boy calmly said in front of cameras that he participated in four killings while drugged and under threat.
But his family has said they don't believe the confession. A close relative, who asked not to be quoted by name for fear of retaliation, has told The Associated Press that he believes Jimenez was an errand boy for criminals, not an assassin. The relative said the boy might have confessed to the killings to show off.