A two-year-old boy found collapsed at a house died of blunt force trauma to the abdomen, initial tests have concluded.
Northamptonshire Police said the provisional results of a post-mortem examination carried out at the weekend revealed the youngster suffered internal injuries.
Further tests are being carried out before a final cause of death is confirmed, the force added.
Officers were called to a house in Arthur Street, Northampton, at 12.45pm on Friday, where they found the toddler unresponsive.
He was taken to Northampton General Hospital by paramedics but died later.
Raphael Kennedy, 30, was arrested in connection with the death and charged with murder. He is due to appear at Leicester Crown Court on Wednesday.
Detective Chief Inspector Ally White, from the East Midlands Special Operations Unit Major Crime Team, said: "The death of a child is devastating and we are doing all we can to establish the full circumstances of this incident.
"As people would expect, our specially-trained officers are offering support to the child's family, who have requested privacy at this time."