A child aged under two has died after he was discovered injured in a house where two adults were found dead.
Police and the air ambulance were called to Well Yard, off Town Street, in the village of Holbrook, near Belper, Derbyshire, at about 1100 BST.
The bodies of a 44-year-old man and a 38-year-old woman were found at the house, police confirmed.
The injured 23-month-old boy was taken by air ambulance to the Royal Derby Hospital but died later.
Officers said they were called to the house by a concerned neighbour.
'Unfair to speculate'
The rural village has been cordoned off but officers have denied reports of a gunshot being fired.
Police said they are not looking for anyone else over the incident.
Assistant Chief Constable Dee Collins said the man and woman were found dead inside the property and the boy died later in hospital.
She said: "The bodies of a man and a woman were found in the house.
"An injured child was taken to the Royal Derby Hospital but died later.
"We have launched a murder inquiry but we are not looking for anyone else."
She added: "Our thoughts go out to the family and friends of those who have died today.
"We are conducting a detailed inquiry and until we can be sure of what happened at Well Yard this morning, it would be unfair to speculate about the circumstances surrounding these deaths."
Post-mortem examinations are due to be carried out later to establish how the three died.