A 15-year-old boy has been stabbed to death after a fight between two groups of youths.
Four teenagers have been arrested on suspicion of murdering the boy, who was attacked in Birmingham.
He was rushed to hospital but, despite the efforts of medical staff, died from his injuries, West Midlands Police said.
Detective Inspector Tom Chisholm, who is leading the investigation, said: "We are aware that there were groups of young people in the area, some of whom may have left the scene before the police arrived.
"There is some suggestion that this was a fight which has tragically gone too far. I would ask that people who were in the park or anyone else with information to contact us immediately."
Relatives of the murdered teenager, who has not yet been identified, are being supported by family liaison officers. Two 16-year-olds, a 15-year-old and a 17-year-old are in custody at separate police stations.
Officers were called to the scene near Oaklands Park in South Yardley at around 6pm on Monday. The four teenagers, all from Birmingham, were arrested nearby.
Mr Chisholm said: "This was a tragic incident and our deepest condolences go out to the young man's family. A murder investigation was immediately launched. We have a team of detectives working with local officers to gain an understanding of what happened."
He added: "We are very well experienced at supporting people who are reluctant to share important information which helps solve serious crimes like this. If anyone finds themselves in that position I would urge them to contact me personally."
A search of the area, which was initially carried out under floodlights, has resumed. CCTV is also being studied and police have increased patrols in the area to reassure local people.