Post-mortem examinations were being carried out on the bodies of two little girls and their parents who were found dead in their family home, English police said.
The toddlers' father is believed to have slit the children's throats and their mother's before hanging himself.
Police were called to the semi-detached house in Provost Street in the New Forest village of Fordingbridge in England at 11.20am on Monday after the shocking discovery.
The family were named locally as Andrew and Vicki Case and their daughters Phoebe, two, and Nereya, one.
Neighbours said they believed Case slit the throats of his wife and the two young children before killing himself.
The ambulance service said Case, 32, was found hanged.
The toddlers' grandmother, who usually looks after them on Mondays, went to the house after phoning the family repeatedly and receiving no answer, a neighbour said.
Alayna Brooks, 53, rushed over to the house herself after the grandmother found the bodies. She told how the woman emerged screaming "all my babies are dead".
Ms Brooks said she went inside and found Mrs Case, in her 30s, dead in the dining room with a plastic bag over her head. She then went upstairs and found the little girls dead on a bed. It is unclear where the man was found.
A cordon was set up around the £250,000 (€298,282) property, which lies in a pretty street of terraced and semi-detached houses, some with riverside views.
Scenes of crime officers later used a recovery lorry to take away a blue Land Rover Freelander that had children's toys inside and an 08 blue-grey Volkswagen Golf with two child seats inside.