A woman working at Mother Teresa's Missionaries of Charity in the eastern Indian state of Jharkhand has been arrested for allegedly selling a 14-day-old baby.
Two other women employees from the centre have been detained and are being questioned about other possible cases.
Police took action after the state's Child Welfare Committee (CWC) registered a complaint.
The BBC has attempted to contact the charity for comment.
"We have found out that some other babies have also been illegally sold from the centre," a police official told BBC Hindi's Niraj Sinha."We have obtained the names of the mothers of these babies and are further investigating."
Police also recovered 140,000 rupees ($2,150; £1,625) from the centre, which is located in Jharkhand's capital, Ranchi.
Missionaries of Charity, which has many centres in India, runs homes for unmarried pregnant women. But they stopped arranging adoptions three years ago when they were unable to comply with new adoption laws.
Desperate, childless parents in India have been known to buy babies illegally because the country has tough adoption laws and a long waiting list for those who want to adopt. For instance, during the 2015-2016 financial year, there were only 3,011 legal adoptions in India against a waiting list of 12,000 couples.
"We are currently investigating the sale of a newborn baby to a couple in Uttar Pradesh for 120,000 rupees," CWC chairman Rupa Kumari told the BBC."But the couple was told that the money was for hospital expenses."
The baby, a boy, was born to a young woman who arrived at the charity on 19 March, and was sold to the couple on 14 May, Ms Kumari said.
She added that the committee should have been informed when the pregnant woman was taken to the hospital. She said the committee had discovered that other babies were sold to people in different cities for 50,000-70,000 rupees.
The CWC has transferred 13 pregnant women who were living in the missionary's Ranchi centre to a different location.
Nobel-laureate Mother Teresa, who died in 1997, founded the Missionaries of Charity in 1950.
The sisterhood has more than 3,000 nuns worldwide. She set up hospices, soup kitchens, schools, leper colonies and homes for abandoned children.
