A three-year-old boy died after being hit by a tractor on a farm.
Ben Norris, known as Benny, was taken to hospital but died of his injuries.
Police were called to the rural property near Whatley, Frome, at around 10.40am on Tuesday.
Ben was treated at the scene by paramedics and taken by ambulance to the Royal United Hospital in Bath, but later died from his injuries.
DS Rachel Whelan, from Avon and Somerset Police, said: "Our thoughts are very much with the boy's family, who have been left distraught by his death.
"A multi-agency investigation is under way in accordance with national rapid response protocols following the unexpected death of any child. At this time we are treating this as a tragic accident."
A spokesman for the Health and Safety Executive said they have also launched an investigation.
"As always, when a fatality is involved the police are leading enquiries at this stage," a spokesman said.
"An inspector has been to the scene. At the moment it is very early days."