A 13-year-old hitman was found dead with his body showing signs of torture, Mexican police have said.

Jorge Moreno was a self-confessed killer on the payroll of the bloodthirsty powerful Los Zeta drug cartel.

Arrested by police last month, Moreno said he took part in at least 10 killings, murdering three people with his own hands.

He confessed to killing a taxi driver and taking part in the murder of a woman, whose body was set alight while he watched.

Despite the confession, police was forced to free Moreno and return him to the care of his mother because of his age. Criminal responsibility is deemed to start at the age of 14.

His body was found on a roadside near the town of Morelos, Zacatecas state, in central Mexico the day after his release. Beside him were the corpses of four unidentified women and a man. One woman was believed to be Moreno's mother.

All bodies displayed signs of torture, said police.

"It would have been better for him if we'd been able to keep him in custody," said state prosecutor Arturo Nahle.

It has been esteemed that 20,000 minors fill the ranks of Mexico's organised crime gangs.

In 2010, 12-year-old Edgar Jimenez, dubbed El Ponchis (the Cloaked One) was arrested.

"I participated in four executions but I did it drugged and under threat that if I didn't they would kill me," he said.

El Ponchis confessed to taking part in the beheading of four people and was sentenced to three years in jail.