MEXICO CITY -- A child welfare official in northern Mexico took at least nine babies from poor or drug-addicted mothers and offered them to adoptive parents in exchange for payments ranging from $5,000 to $9,000, authorities said Friday.

Raul Ramirez, the head of the government human rights commission in the border state of Sonora, said the scheme apparently went on for years and may involve many more children.

"They searched for vulnerable mothers, poor people or those who had problems of drug addiction, and took away their babies and offered them in adoption in return for money," Ramirez said.

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Three of the babies have been identified and recovered, but Ramirez said "there may be many more, from years back, and some of these children could be 20 years old by now."

The problem, he noted is that "the children have developed affection for their (adoptive) parents, and now they're crying for their parents."

All the children were apparently adopted by Mexican couples.

The state prosecutors' office said the main suspect, Vladimir Arzate, 30, worked in the office of the state prosecutor for child protection. The office had the power to take in at-risk children, but would have had to turn them over to a child welfare agency.

Instead, Arzate is accused of working in collusion with a doctor, who would deliver fake birth certificates for the stolen babies under the adoptive parents' names, listing them as the biological parents.

Ramirez said many of the babies went to middle-class or upper-middle-class families desperate to adopt and avoid the lengthy, complex process that rules adoptions in Mexico.

The prosecutors' office said 16 arrest warrants have been issued, which appear to cover some of the adoptive parents.

Since none of the children appear to have been abused, prosecutors are charging the suspects with child trafficking or child theft.

Those charges are punishable by 15½ to 40 years in prison.

The question remains as to how the scheme was allowed to operate for so long. Apparently, few of the biological parents had contacted authorities seeking the return of the children.

Ramirez said one possible biological grandmother had contacted the office. Local media cited the story of a farmworker whose son was taken from a local hospital after she brought him in for treatment of heatstroke.

Ramirez suggested more officials or doctors may have been involved.

"The big losers here are the children," said state Attorney General Carlos Navarro.