Police are investigating whether violence that left one teenager shot dead and another two with knife injuries was gang related.
Officers found the dying boy, 17, in an estate stairwell in Peckham, south London, at about 1630 GMT on Wednesday.
Police also discovered a 17-year-old with a stab wound at Gannet House and suspect the incidents are linked.
Officers were later alerted to the third victim, 18, after he went to a south London hospital.
Three teenage males, two aged 17 and one 18-year-old, have been arrested in connection with the incidents. All three remain in custody.
A post-mortem examination revealed the teenager found in the stairwell was shot in the neck and chest.
Officers earlier believed he was also stabbed, but tests showed this was not the case, the Metropolitan Police said.
Speaking at the scene on Thursday, Det Ch Insp Gordon Allison said officers found the 17-year-old - who is yet to be formally identified - on the fourth floor of the property.
"They tried to give him first aid and called the ambulance - unfortunately the young man died at 5.15pm.
"As a result of that we are now carrying out a murder inquiry."
Det Ch Insp Allison said it was too early to say for sure whether or not the violence was gang-related.
But he added: "Obviously that is a line of inquiry that my team will be pursuing."
Police also made an appeal for information, stressing that the Trident unit, which investigates gun crime in the black community, "leads the field" in witness protection.
The dead teenager is yet to be named and no weapons have been recovered.
The 17-year-old stab victim was taken to a south London hospital where he remains in a stable condition
The 18-year-old stab victim was treated before being allowed home.
A police cordon has been put in place around the estate and officers have not let residents back into Heron House, where the victim died.
Residents who lived close to the site of the shooting said they heard a large bang coming from Heron House.
"I thought someone was setting off an early New Year's Eve firework," said one woman.
But a man who lived nearby said he knew instantly it was a gunshot.
He later came across the crime scene as he tried to leave the block of flats.
"I live on the fifth floor and I went out to the lift.
"Then I saw a lot of blood outside the lift on the floor. The police were already there," the man said.
Footballer Rio Ferdinand called on the government to do more for young men in light of the killing, which happened less than a mile from his birthplace.
He wrote on Twitter that Peckham was "again in the news for the wrong thing" and called on the government to "sort this out please".
Ferdinand added: "If the youth clubs were there then maybe kids would have something to do to keep them occupied."
Camberwell and Peckham MP Harriet Harman visited the scene on Thursday.
She said: "My deepest sympathies are with the parents and the family of the young man who has been killed.
"It is vital that those who committed this brutal crime should not get away with it.
"The overwhelming majority of young people in this area are decent, hardworking people who want to make the most of their lives and people are dismayed that once again a tragedy has cast a shadow over Peckham."