A 13-year-old girl who was stabbed in a "random, isolated attack" dialled 999 as she lay dying from her injuries, police said tonight.
Casey-Lyanne Kearney called police and a passer-by phoned for an ambulance, but despite their efforts she died in hospital hours later.
A 26-year-old woman who was arrested on suspicion of murder remains in police custody. It has been confirmed that she was "known to a number of local public agencies".
John Willis, who comforted Casey-Lyanne, was one of the first people to find the dying girl in Elmfield Park, Doncaster, South Yorkshire.
He was alerted to her plight by his two sons who saw her lying on the ground in a semi-conscious state.
He said: "I was expecting to find someone having a fit, she was on the ground lying on a coat, another woman was with her making sure she was cared for.
"We got her in the recovery position. She didn't move a lot and she didn't speak. Her breathing was very shallow and she was very pale.
"The police arrived very promptly and took over the situation. I was very upset, it was very shocking."
Senior investigating officer Detective Superintendent Terry Mann of South Yorkshire Police said officers were attempting to establish the teenager's movements after she left her home in Rossington at around midday yesterday.
He said Casey-Lyanne was intending to catch the bus to Doncaster town centre where she was going to meet a friend.
He said: "South Yorkshire Police have launched a murder investigation after a 13-year-old girl died last night at 9.25pm in Doncaster Royal Infirmary (DRI) following what appears to be a random, isolated attack.
"We know that Casey-Lyanne Kearney left her home in Rossington around 12 noon yesterday and caught the number 55 bus into town to meet a friend.
"We are working to ascertain Casey's movements after leaving home.
"At 1.18pm Casey called police from inside Elmfield Park saying she'd been stabbed.
"She was treated at the scene by police and paramedics before being taken by ambulance to DRI."
Mr Mann said Casey-Lyanne was 5ft 3in with long, straight, ginger hair.
She was wearing a black waist-length coat yesterday, along with blue jeans and knee-high boots. She was also carrying a black Bench shoulder bag with purple writing.
He appealed for any members of the public who were in or near the park at lunchtime yesterday, and either saw anything suspicious or Casey-Lyanne herself, to contact police.
A specialist incident room has been set up which can be contacted on 01709 443510, or people can also call Crimestoppers anonymously on 0800 555 111.
The park remained sealed off today and blue police tape was attached to railings and trees.
A white forensic police tent could be seen in the park - just a few metres from a play area.
Locals spoke of their shock at the stabbing.
One woman, who was passing the scene with her 13-year-old daughter, said she no longer allowed her to go into the park.
"It is not a very good park, to be honest. There have been a number of incidents in there, it is not like when I was a child," she said.
She explained that her daughter had come across a drunk woman when she was walking close to the park yesterday afternoon.
The teenager said: "We were just walking and we decided to run. She was walking past and she asked us why we are running. She started calling us names."
A local shopkeeper, who gave his name as Raja, said: "They should put CCTV cameras in the park. A few customers have been saying that."
He added: "It is such a shock. It is really sad. You see kids playing football in the park. It is the first time we have had a stabbing around here."
He said five or six years ago a man set himself alight in the park.
Doncaster Council chief executive Jo Miller said: "We are deeply saddened by this tragic event and our thoughts are very much with family and friends.
"This incident is in the very early stages of a police-led murder investigation and we would urge anyone with any information to come forward and talk to the police.
"We can confirm that we are all working hard to piece together what happened yesterday and also gathering any information we can.
"The person in custody was known to a number of local public agencies and we are working together to establish all the circumstances involved with this case."
She said the investigation was at an early stage and any member of the public who had any information should come forward.