Detectives investigating the murder of a boy have narrowed down his origin to a region of south-west Nigeria, 16 months after his torso was found in the Thames.

Samples from his bones were found to contain traces of a type of rock found in a region between Benin City and Ibadan. Scotland Yard detectives, who believe he was brought to London and killed as a human sacrifice, will travel to the area to search for the his killer.

Adam, who was discovered near Tower Bridge in September 2001, was thought to be aged between four and seven, but other than that little was known about him.

A team from Scotland Yard travelled to Nigeria and took 100 samples from soil, rocks and meat in a 10,000 square kilometre area. Professor Ken Pye, a forensic geologist, said the boy could not have come from 95 per cent of Nigeria. Further forensic work is being done on a concoction found in his lower gut.

Detective Inspector Will O'Reilly said: "A few months ago we were working on Adam being from somewhere in five countries in West Africa and to narrow it down is remarkable."

Detectives have not eliminated from their inquiries Joyce Asaguede, 31, who was arrested in Glasgow in July. She was not charged and was returned to Nigeria after a failed asylum application.