The person or people who killed two teenage boys in Milton Keynes last night remain at large.
Seventeen-year-olds Ben Gillham-Rice and Dom Ansah were both stabbed to death just before midnight at a private house party in Emerson Valley.
Two other young men aged 17 and 23 were also stabbed but their injuries are not life-threatening, say police.
The party began early yesterday evening for 15 to 20 young people in a detached house in Archford Croft. Neighbours noticed a birthday banner strung above the front door and heard music.
All the victims were invited guests and all part of the same friendship group of the girl who was hosting the party. Police have confirmed there were no gatecrashers.
investigating officer Detective Chief Superintendent Ian Hunter said this afternoon: "At this early stage I believe the people involved in this incident are all likely to have known each other."
It is still a mystery why four people from the same friend group ended up being stabbed, two of them fatally.
A neighbour said: "We didn't hear any trouble until just before midnight when there was shouting and screaming. That must have been after the stabbings took place.
"It seemed a nice, friendly house party. Why on earth would people be carrying knives to that? It is terrifying that a celebration can turn to double murder in a matter of minutes."
It is understood the culprit or culprits ran off afterwards. Police searched the area with a helicopter and tracker dogs but no arrests have yet been made.
DCS Hunter said: "Clearly this is a dreadful incident and we are in the very early stages of the investigation. We have deployed significant resources and it is, and will continue to be, a fast-moving investigation led by our Major Crime Unit.
"Our thoughts go out to the families of the victims and we have deployed specially trained officers to support them.
"We remain on the scene of Archford Close in Emerson Valley and a scene has been cordoned off ... This is likely to remain for several days.It is very important that our investigations tem receive all relevant information so I urge anybody who can help, no matter how insignificant you think this may be, to come forward."
Police want to hear from anyone who was in this area of Emerson Valley between 11pm last night and 1am this morning. there will be an increased police presence on Emerson Valley and officers are encouraging the public to stop them and talk to them if they know anything.
Anybody who can help should call 101 quoting reference number 1761 of 19th October.