A 14-year-old girl is critically ill after she was repeatedly stabbed in a "frenzied" and unprovoked attack.
Jessica Knight suffered several stab wounds in the attack
Police are warning people in Chorley, Lancashire, to take extra precautions until the offender has been caught.
Jessica Knight was found with multiple wounds by a passer-by in the town's Astley Park on Monday afternoon.
She was given first aid at the scene and taken to Chorley and South Ribble Hospital, where she underwent four hours of surgery.
Officers have said the attack was "frenzied" and are appealing for witnesses to help piece together events leading up to the attack.
They have cordoned off a large area of the park, which is a popular spot with dog walkers and is also used as a thoroughfare by local people.
It is set in Astley Village, an affluent area north of Chorley which has council offices on one side and sheltered housing on the other.
Local people are being warned to be on their guard
Jessica had gone into the park at about 1700 GMT, after she had been seen on CCTV cameras at a shop in Astley Village.
Police said members of the public heard screams and at about 1714 GMT a cyclist found her unconscious and "covered in blood" near Astley Hall, about 80m (262ft) from Hallgate Road.
Det Supt Steve Brunskill, of Lancashire Police, said: "This is a particularly nasty assault on a defenceless young girl.
"I am appealing for anyone who was in the Astley Village or Astley Park area between 1630 and 1730 GMT yesterday to contact the incident room at Chorley.
"We have a couple of people in the buildings surrounding the scene who have overheard some screams - other than that, we do not have any witnesses.
"I am particularly interested to hear from anyone who may have seen any suspicious behaviour or who has noticed anything in the area recently that has aroused their suspicion.
"At this early stage of the investigation we are unsure of the motive and would urge people in the area to take precautions for their own safety until this dangerous offender, or offenders, are caught."
Officers are with Jessica's family at hospital and are waiting to speak to the teenager to try to find out what happened.
Mr Brunskill added: "She underwent an awful lot of surgery and she was in a bad way. She is stable now."
He said there was nothing in her background to suggest a motive and it was not a sex attack.
"The family are very distressed. As far as we are aware, this is a perfectly normal, happy child. It's mind-boggling how this can occur in this area," he said.
Extra officers, including mounted police, are currently on patrol in the area.
Police will also be visiting local schools, and a mobile police station will be set up in the Astley Village area.
Jessica was wearing a black hooded jacket with fur lining, blue jeans and white trainers when she was attacked.
Jean McGrath, head of Parklands High School in Chorley, said Jessica was "a lovely, hard working girl".
"I spoke to all our pupils today telling them of the incident, thinking and praying for Jessica and reminding them to be extra vigilant with regard to their own safety," she said.
Laura Lennox, a councillor with Astley Parish Council, said: "It's not a path I would use on my own at night. It is very tragic that this should happen."