Mexican authorities have captured an alleged drug trafficker from the Caballeros Templarios (Knights Templar) cartel who officials say is being investigated on suspicion of kidnapping and murdering children in order to harvest their organs for sale.

The security secretary for Michoacán state, Carlos Castellanos, said Manuel Plancarte Gaspar was arrested last week in possession of a stolen vehicle and drugs, and was suspected of involvement in a network that killed children in order to extract and sell their organs.

Castallenos gave few details of the investigation, but one of the leaders of a vigilante movement in Michoacán formed to combat the Caballeros Templarios responded to the news by saying locals had long known that organ trafficking "was part of the business of these people".

José Manuel Mireles told MVS radio that vigilantes he led in the town of Tepalcatepec once stopped a suspicious-looking truck driving through the area and discovered kidnapped children inside a refrigerated container. He said the truck had taken a wrong turn on its way to the port city of Lázaro Cárdenas, long a hub of cartel activity.

"The children were still alive and wrapped in blankets," Mireles said. "They were all from the same school in Mexico City and their parents had let them go because they were told they were going on a trip to the beach."

Mireles said the vigilantes got in touch with the children's parents who travelled to Tepaltepec to take them home. He did not specify the number of children involved or the date when he claims the discovery happened, though the vigilante movement has only been active in the area for the past year.

He said he had no detailed knowledge of Plancarte Gaspar or his alleged involvement in such a practice. The detainee is reportedly a nephew of Enrique "Kike" Plancarte Solís, one of two Caballeros Templarios leaders still at large in the midst of a major federal offensive against the cartel, whose spiritual leader Nazario Moreno was killed this month.

Mexican cartels in general, and the Caballeros Templarios in particular, are known to pursue a broad portfolio of illegal activities beyond drug trafficking, ranging from kidnapping to sexual slavery rackets to exporting iron ore to China.

Accounts of organ trafficking, however, are usually dismissed by experts as mere rumours, even when involving the country's most sophisticated and ruthless cartels, given the amount of medical infrastructure and fine-tuned complicity with hospitals and doctors required to get organs to patients awaiting transplant on time.

Alejandro Hope, a former government official and an expert on Mexico's security crisis, said he remained doubtful that any cartel was involved in organ trafficking in any substantial way. "There might be a case here and there," he said. "I don't see that it could exist on any major scale."