Detectives launched a murder inquiry after a 15-year-old boy died in hospital following an attack at a party.
Police and paramedics were called to reports two teenagers had been assaulted outside a flat in Gorringes Brook, Horsham, West Sussex, at 2am on Friday.
They found the 15-year-old with life-threatening head injuries and a 16-year-old with less serious head injuries.
The younger boy was taken to hospital but died later, Sussex Police said.
Police have arrested an 18-year-old local man in connection with the attacks.
Detective Chief Inspector Adam Hibbert said: "This was a very serious assault involving persons who had been attending a party in Gorringes Brook.
"We believe those involved may know each other and this does not appear to have been a random stranger-type attack."
He urged anyone who attended the party to call Crimestoppers on 0800 555 111 or Sussex Police on 0845 6070999.
