|A frame from the video, believed to have been posted online by a rival cartel, shows young boys posing with guns.
A gang of teenagers employed as "hitmen" are suspected of being responsible for dozens of killings, Mexican police said.
Police detained a minor on Friday accused of working as a gunman for a drug cartel after shocking videos and photos were posted online by a rival gang, and local media reported that police were also seeking a 12-year-old hitman nicknamed "El Ponchis," (the cloak) but there was no confirmation of that from prosecutors.
Pedro Luis Benitez, the attorney general of Morelos, told a local radio station that police had detained a minor who allegedly worked as a gunman for a drug cartel and were looking for another.
He did not say whether the minor who was detained or the one being sought had appeared in the online photos or video. He also did not give the age of the suspects, but he implied they were young enough to be playing with toys.
"It is easy for them (criminals) to give them a firearm, making it appear as it if were a plastic weapon and that it is a game, when in fact it is not," Benitez said.
There are reports that El Ponchis is a sadistic killer whose preferred method of killing is to slit the throats of his victims. He is believed to be employed by South Pacific Cartel, operating in the state of Morelos, just outside Mexico City.
Young and dangerous
The material posted online shows boys believed to be as young as 12 years old, pose for the camera with guns and corpses.
"Where more disorder exists, there will be more violence"
Genaro Garcia Luna, Mexico's secretary for public safety
One video, briefly posted on YouTube, showed a youth, apparently in his teens, confessing to working for a branch of the Beltran Leyva cartel. While the authenticity of the video could not be determined, cartels in Mexico frequently post such interrogation videos to expose their rivals' crimes.
One youth is recorded telling an unseen questioner that his gang was paid $3000 per killing.
"When we don't find the rivals, we kill innocent people, maybe a construction worker or a taxi driver," the youth says in the video.
Felipe Calderon, the Mexican president, who launched the offensive against cartels in 2006, acknowledged several months ago that "in the most violent areas of the country, there is an unending recruitment of young people without hope, without opportunities".
Suspects under 18 are prosecuted in a separate legal system for young offenders for most crime in Mexico. But there are growing calls for that system, and the nation's overcrowded adult prisons, to be revamped.
Mexico has more than doubled the number of people in federal prisons in the last two years as part of the country's crackdown on drug cartels, a security official said on Friday. While the federal prison system had about 4,500 inmates in 2008, there are now 11,000.
"Where more disorder exists, there will be more violence," Genaro Garcia Luna, the secretary for public safety, said. "The penitentiaries can be places where not only do people complete their punishments, but where future delinquent conduct is prevented".
He cited one prison in particular, Islas Marias, which has seen its inmate population quadruple since 2006. Located off the coast of Sinaloa state, the prison now houses 3,946 inmates.
"We are trying to abate the deficit of space and modernise our prison system," he said.
More than 28,000 Mexicans have been killed since late 2006 in drug-related violence, and 2010 is on track to be the bloodiest so far.