Two 14-year-olds are among three teenagers arrested after a boy, 17, was stabbed to death in a "targeted attack" in Cambridge.
A 17 year-old was also arrested after the attack at Logan's Meadow, a small nature reserve next to the River Cam.
Police were called to the scene by the ambulance service just before 3pm.
Despite the efforts of emergency workers, the teenager was declared dead, Cambridgeshire Constabulary said.
Detective Superintendent Carl Foster, from the Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit, said police are following several leads but believe the attack was "targeted".
Appeal to public
He said: "We believe there were members of the public at the scene who spoke with paramedics but left before our officers arrived. I am directly appealing for these people to make contact with us please.
"I would appeal to anyone with information to get in touch with us as a matter of urgency."
Police are appealing for anyone with information to get in touch by calling 101.