The initial investigation concluded the baby was choked to death.
Pham Thi Xuan at the police station
The woman first confessed to the police she killed the baby following advice from a fortune-teller who said her granddaughter was a demon and would threaten the family's life.
But later she insisted that was an accident when she dropped the baby on the floor and fell on her.
The baby was killed on the evening of November 25 just after her father left their home in Bim Son Town to pick up his older child from an extra-class.
The father, Le Huu Thuan, then returned home to find his mother lying unconscious at the door. He called his wife, who was working on the computer in the next room but knew nothing about the incident, to take her in.
Xuan then woke up and told her son and daughter-in-law that the baby had been kidnapped by a woman and a man who attacked her with anaesthetic and took the baby away on their motorbike.
After meeting with the police that evening, Xuan put the body of the baby in a bag and left it at a dustbin some few hundred metres from their home.
The baby's body was then found two days later at a dumping site 10 kilometres away from her home by a metal scrap collector.
Deputy director of Thanh Hoa Provincial Department of Public Security, Khuong Duy Oanh, said on Wednesday that finding out the cause of the baby's death was an important step in the investigations.
"We're further investigating the killer's motive,"Oanh said.
