Four people have been arrested over the killing of a teenager who was stabbed at a house party in Essex.
Jay Whiston, 17, was attacked at the event in Colchester and died a short time later in hospital.
It is believed the party on September 8 got out of control after details were posted on Facebook.
Detectives arrested two men, a woman and a girl at a house in the Colchester area.
The men, aged 51 and 18, the 42-year-old woman and the girl, who is 17 and from Clacton, are being questioned at a police station in north Essex.
Five people from Colchester are on police bail after they were also arrested in connection with the killing.
Detective Chief Inspector Simon Parkes said: "This is very much a live investigation where we continue to benefit from the support of the public."
He added: "Today we have arrested four on suspicion of assisting an offender. We remain convinced that there are people who have yet to contact us, who know what happened and are protecting those responsible.
"It is those people we would appeal to come forward."
Since the teenager's death, his mother, Caroline Shearer, has launched a campaign to fight knife crime .
She has opened a charity shop in Colchester to raise funds for her campaign, entitled Only Cowards Carry.
Mrs Shearer has also called for "weapons awareness" to be taught in schools.
She said: "It's no good teaching them these lessons once these children have found out what path they want to take in life.
"The younger the child you can get to, the more chance you've got of changing the mindset of why our children are carrying knives."