An American-born 14-year-old is being held by Mexican authorities for allegedly helping a drug gang torture and kill their victims.

The teenager known as "El Ponchis" was under the guard of armed Mexican soldiers in Morelos when he allegedly told reporters that he helped a Mexican drug gang behead four people.

Mexican officials say they arrested the youth, whose first name is Edgar, at an airport south of Mexico City on Thursday with a 19-year-old sister. The sister is accused of helping him dump the bodies.

Authorities revealed the teenage boy was caught with two mobile phones that held photographs of tortured victims.

Many youths have been used by drug cartels in their bloody battles against the government and each other, but the story of El Ponchis may be the most shocking.

The boy revealed to the media that he participated in four executions but alleged he was drugged and his life was being threatened if he did not cooperate.

Edgar told reporters that he was kidnapped at the age of 11 and forced to work for the Cartel of the South Pacific, a branch of the splintered Beltran Leyva gang, and that he had participated in at least four decapitations.

The boy's sister said they were headed for Tijuana, where they planned to cross the border and seek refuge with their stepmother in San Diego.