(CNN) -- The trial is being held under strict security measures.
The defendant was transported to the courthouse compound in a military convoy. Because he is a minor, the public is not allowed inside the courtroom in Cuernavaca, Mexico. Only the judge, defense and prosecution attorneys, family members, and a human rights observer are allowed inside.
The defendant, known as "El Ponchis" ("The Cloak") is an American citizen who is 14 years old.
A video provided to CNN by the Mexican military shows the San Diego native after he was captured near Mexico City in December, as he allegedly was trying to flee. In an on-camera interrogation by Mexican military authorities, the youth admitted to brutally killing people -- the victims all were beheaded.
The video shows a military interrogator asking the slim teenager with curly hair several questions.
"How many have you killed?" he asks.
"Four," responds the accused, who seems calm and collected.
"How did you execute them?"
"I slit their throats."
A YouTube video that circulated last year purportedly showed the teenager beating a man with a two-by-four while the man was tied at the wrists and hanging from the ceiling, as other young people watched.
Juan Carlos Castro, a spokesman for the juvenile court holding the trial, says the charges against the teenager go well beyond what he admitted on video.
"This teenager is accused by the state attorney's anti-drug unit of crimes related to drug trafficking -- specifically cocaine and marijuana -- illegal possession of military weapons, and violations against federal organized crime law with the objective of committing kidnappings and aggravated murder," Castro said.
Raul Diaz Garcia, the state human rights commissioner for Morelos state, where Cuernavaca is located, is attending the trial. Diaz said he had an opportunity to speak with the defendant.
"He seemed coherent and looks clean. He spoke with us without any trouble and was smiling. We told him that we were there in case he had any questions (about his rights)," Diaz said.
"El Ponchis" is not the only minor in Mexico allegedly involved with drug cartels. Social media pictures have surfaced in the last year showing teenagers and young men holding high-caliber assault weapons and wearing masks.
Six youths were captured in October in a town about 30 miles south of Mexico City. Graffiti in the house where they were hiding linked them to the so-called South Pacific Cartel, a splinter group that formed after cartel leader and drug lord Arturo Beltran Leyva was killed in a shootout with the Mexican military in December of 2009.
Veronica Morales, the director of the Mexican Network for the Rights of Children, said that unfortunately this is not an isolated case.
"According to academic studies, around 30,000 children and adolescents are currently involved or may be involved in organized crime," Morales says.
If found guilty, "El Ponchis" faces a sentence of three years in a correctional facility, the maximum allowed under Mexican law because of his age.
There will be about 60 witnesses testifying at the trial, which is expected to last about three weeks. A juvenile judge will determine guilt and decide on a sentence after hearing all the testimony and arguments by the prosecution and defense. There is no trial by jury in Mexico.
Christian Fragoso, a criminal defense attorney familiar with the case, says that even if prosecutors can prove a long criminal history, they will be restricted by current criminal juvenile law. "Only those crimes committed after reaching the age of 14 can be taken into account. He cannot be found guilty or sentenced for anything he did before," Fragoso says.
"El Ponchis" has been held at a state correctional facility in Cuernavaca since he was captured last December.
CNN's Krupskaia Alis in Mexico City contributed to this report.