Two 17-year-old boys have been stabbed to death at a house party.
Police and paramedics were called to a house in Archford Croft in Milton Keynes at about midnight on Saturday.
The teenagers have been named locally as Dom Ansah and Ben Gillham-Rice, as relatives said their "hearts are broken".
One of the boys died at the scene and the other in hospital. Thames Valley Police said no arrests had been made in the double murder inquiry.
A 17-year-old boy and a 23-year-old man were also hurt and were taken to hospital with serious but not life-threatening injuries, the force said.
Det Ch Supt Ian Hunter said the stabbings happened "at a private house party" and those involved in the violence "are all likely to have known each other".
He said police believed the victims had been invited to the party, which was attended by 15 to 20 people.
Officers are expected to remain at the scene, which is on a cul-de-sac in a housing estate in the Emerson Valley area, for several days.
Stains of what appeared to be blood could be seen on the front door of a house inside the police cordon.
Two of Dom Ansah's cousins laid flowers at the cordon on Sunday afternoon.
"He's come here with his long-time best friend since childhood, comes to a party and both of their lives just got ripped away from them," said one, who did not give her name.
"He was just so respectful to like his family and friends. Many, many people's hearts are broken."
A neighbour said she believed the gathering was a party for a teenage girl living in the house, while others said a birthday banner had been hanging at the door earlier in the evening.
She said she saw police cars and ambulances at the scene after being woken by her husband during the night.
"I was so terrified," she said. "I've never seen such a scene until today."
Another neighbour, who lives in an adjacent cul-de-sac, said: "This gang of kids have been hanging around Archford Croft, it's all gang-related.
"I think it's just because there was a house party and then the trouble started from there."
Milton Keynes South MP Iain Stewart said he would offer assistance to the families affected.
I am shocked by this double murder in my constituency. I offer my deepest condolences to the families affected and am available to offer them assistance. I shall also be speaking to the police to understand the circumstances behind the crimes.https://t.co/RRgun2CPRK— Iain Stewart MP (@iainastewart) October 20, 2019
Det Ch Supt Hunter appealed for "anyone who has any information which could help with our inquiries or anyone who witnessed any suspicious activity" to come forward.
"Thames Valley Police is in the early stages of a double murder investigation after two teenage boys have tragically died in this shocking incident," he said.
"Even if you think details may be insignificant, please come forward and speak to police."