Thirteen-year-old Billie-Jo Jenkins was found dead by her foster father when he returned with two of his other daughters to the family home in Hastings, East Sussex, on Saturday afternoon.
Billie-Jo, described by neighbours as a "caring, loving girl", had been painting the patio doors when she was attacked.
Detective Superintendent, Jeremy Paine said: "The awful thing that confronted him and his daughters on their return is almost unimaginable.
"This was a vicious, ferocious attack on a young girl in her home."
A post-mortem examination carried out yesterday revealed that the teenager died from severe head injuries resulting from several blows to the skull.
The murder weapon is believed to be an 18-inch metal spike found by the killer in a store room in the back garden. It was discovered by police lying next to Billie-Jo's body.
There were no signs of any sexual attack and the house had not been broken into.
Officers are conducting house-to-house inquiries in the area and will be speaking with Sion and Lois Jenkins, their four daughters, aged 12, 10, nine and seven, and Billie Jo's natural parents, who live in Barking, east London.
Detectives are also trying to track down a man in his 40s with prominent birth marks or scars seen near the family's home on Saturday. Neighbours have told police he was acting strangely and asking around for accommodation. Police say the attack will have left the killer's clothes blood-stained and covered in white paint.
The Jenkins family have been experiencing problems with prowlers since Christmas.Reuse content