An uncle of an 18-month-old girl who was attacked and killed by a dog in Crawley, West Sussex, is being held on suspicion of manslaughter.
Sussex Police said officers were called to reports of a girl suffering head injuries at a property in Cotton Walk, Broadfield, just after midday.
She was taken to East Surrey Hospital in Redhill, Surrey, but died shortly after arrival.
Sussex Police major crime branch has launched an investigation.
The girl, who lived at the property, was with her grandmother, mother and three-year-old sister when the attack happened in the kitchen.
Det Ch Insp Trevor Bowles said: "It is very early in the investigation, but we are trying to establish how this tragedy occurred.
"This is a truly appalling incident which has led to the death of a young 18-month-old child.
"My team will be working tirelessly to ensure that the truth of exactly what happened - and how - is established in the coming days."
A spokesman for Sussex Police said officers were trying to determine the dog's breed but it is thought to be a type of mastiff.
Police said the girl's 32-year-old uncle, who owned the dog, was not in the property at the time of the attack but arrived shortly afterwards and was arrested.
A post-mortem examination on the girl is due to be carried out on Sunday morning.
Forensics officers were seen going in and out of the house, outside which a female police officer stood guard.
Neighbour Connor Maugham, 60, said: "I've seen the dog around, it seemed very placid. I think it's a mastiff, a big dog. Apparently it happened in the kitchen.
He added: "The community is in shock. They are such a nice family, they always keep themselves to themselves. They have got about three children."