"We are doing all we can to learn about what happened that night, the circumstances that led to this attack and now his untimely death.
"I would appeal to anyone who was in the area last night and who may have any information, however small, to come forward."
Superintendent Trevor Botting, East Sussex Division Commander, said: "The community is shocked by this horrific incident.
"We will be talking to residents during the coming days to reassure them and keep them informed about the investigation. I urge anyone who has any information to contact police."
The 13-year-old local boy, who was arrested earlier on suspicion of attempted murder, has now been arrested for murder, along with a 19-year-old local man. They are currently being questioned by detectives.