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.

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.