A fifth teenager has been arrested over the death of an A-level student who was stabbed outside a house party, police said.
Jay Whiston, 17, was knifed on Saturday night at the gathering that was attended by about 100 teenagers in an affluent area of Colchester.
Essex Police said a 17-year-old was being questioned on suspicion of assault after being arrested in the town on Monday.
A 17-year-old boy and two 18-year-old men remain in custody at separate police stations in north Essex being questioned on suspicion of murder.
A 17-year-old boy, who had also been arrested on suspicion of murder, has been released without charge, police said.
It is believed details of the event were circulated on the social networking site Facebook and a number of uninvited guests turned up.
Jay, of Clacton, Essex, was taken to Colchester General Hospital with abdominal injuries but died shortly after his arrival, having been stabbed at around 10pm. A provisional post-mortem examination showed he died from a stab wound.
Essex Police said detectives investigating the murder have set up a dedicated number for witnesses to call.
Anyone with information is asked to contact the incident room on 01245 452882.