The bodies of Jonathan Copley and 15-year-old Rachel Rooney were discovered in a bedroom by the boy's mother, Denise Copley, and his two elder sisters as they returned to their home in Bradford from a party shortly after midnight yesterday.
Det Supt Malcolm Mawson, leading the hunt, said it was a "frenzied and constant attack".
"We believe the weapon used was a knife, perhaps with a blade about six inches long and half an inch wide," he said. Police refused to comment on reports that the victims' throats had been cut. The murder weapon has yet to be recovered. The family were said last night to be "physically and mentally traumatised" after discovering the scene.
A Home Office pathologist and forensic scientists were at the house and post-mortem examinations were being held. A car believed to belong to the parents of Rachel Rooney was also taken away for routine forensic examination. The car, a blue Toyota, was parked outside their home in Queensbury, West Yorkshire.
Rachel, a friend of Jonathan's 15-year-old sister Elizabeth, had been asked to look after the youngster while the Copley family went out to celebrate at a party in Bradford.
A neighbour, 47-year-old Rodney Beck, said it was normally Jonathan's sister Elizabeth who baby-sat, but on this occasion she had asked her friend Rachel to step in. Mrs Copley was said to be separated from her husband, who lived in another area of Bradford.
"We got home at about 1am and the street was crawling with police. This is an absolute tragedy," he said.
One neighbour said she heard a knock on the door around midnight on Saturday and then a car drive away minutes later. "I thought nothing of it at the time but about half an hour later I heard hysterical screaming in the street," said Taranjit Shergill, 22.
"I looked out of the window and there was this girl shouting `Jonathan, Jonathan' and she collapsed in the road. The screaming just continued and then I saw people running to the phone boxes outside the house."
Trainee accountant Sabina Chadun, 23 who had looked after Jonathan, said she was devastated. "He was such a cute lad, very lively, talkative and cheeky. He was lovely," she said. "I was stunned when I heard the news. I just sat down and cried."
Neighbours of Rachel Rooney were also said to be shocked by the death of the "petite, lovely looking girl".
"A number of people have been spoken to in connection with the incident and our inquiries are continuing," said Det Supt Mawson.
"There is no indication at the moment that any sexual assault took place and it is too early to say whether anything has been stolen," he added. Investigations to find the motive were continuing .
Police say they particularly want to speak to a young man seen on a moped- type vehicle outside the house in Southfield Lane, Little Horton, Bradford, between 10.30pm and 10.45pm on Saturday. "He was intermittently blowing the horn of the vehicle and was wearing a crash helmet," said Det Supt Mawson.Reuse content