A woman is believed to have killed two of her children before taking her own life.
Mother-of-five Melanie Stephens, 36, is thought to have suffocated Philip, five, and two-year-old Isaac and then hanged herself at their home in north Wales.
Police forced their way into the property in Trawsfynydd, Gwynedd, at around 8pm last night after frantic relatives were unable to get in.
A North Wales Police spokeswoman said: "Inside the property police discovered the bodies of an adult and two young children. At this early stage it is not possible to state how the occupants died.
"The coroner and next of kin have been informed."
A Home Office pathologist is conducting post-mortem examinations to establish the cause of deaths, although police say they are not treating them as suspicious.
Ms Stephens had lived at the house in Chapel Street for about two years, having moved from the nearby town of Blaenau Ffestiniog.
Philip, known as Pip, attended primary school at Ysgol Edmwnd Prys.
Her three older children are thought to live with their father.
Residents in the village, which has about 700 residents, said the family seemed "happy and pleasant".
A neighbour, who did not want to be named, said: "Obviously everybody in the village is shocked. But nobody knows yet what caused this tragedy.
"I didn't know her very well because she wasn't from the village and I don't think she had many family here.
"But you would see her in the street and she would always say hello. They were a very pleasant family and seemed very happy. It's a terribly sad thing."
Gwynedd councillor Thomas Griffith Ellis said: "The death of a mother and young children is a terrible thing but at Christmas it is even more shocking.
"I didn't know the woman but she lived on the main street next to the chapel, so I would see her coming and going to the shops.
"She always said hello as she passed but never stopped to chat.
"I don't know what has happened, there's been no suggestion why such a terrible thing has happened to a family."