Police believe up to 30 people were present when the murder of 15-year-old Michael Dosunmu at his home in south-east London was planned.
Michael celebrated his 15th birthday days before being shot
Speaking on Crimewatch on Monday Det Ch Insp Fiona Mallon said his murder may have been a case of mistaken identity.
Michael died of multiple gunshot wounds when two gunmen burst into the bedroom of his Peckham home on 6 February.
Detectives said the weapon, a Mac 10 machine-gun, was still on London's streets and could be used again.
Ms Mallon told the programme: "I think there are at least 20 to 30 people who would have been present when it (the murder) was planned, who know who did the shooting and now know where the gun is.
"I am confident this will still be in circulation now."
Police believe the shooting of Michael, who had turned 15 two days earlier, may have been a case of mistaken identity linked to the fatal stabbing of a 21-year-old in an adjacent road three days earlier.
Michael's sister Shakira told Crimewatch how she tried to keep him alive as she called 999 for help.
"I did CPR for him, I did it for the 15 times I put air in his mouth - nothing. I did it again - I was just screaming.
"I said, 'Michael you can't leave me like this. Michael, Michael, Michael - wake up, wake, wake up', but he didn't respond."
His father, Rasak, said his wife, also called Shakira, was visiting Nigeria when the shooting took place and he could not tell her Michael was dead until she returned.
"What goes around will turn around one day. It might be Michael today. It might be their own family tomorrow," he said.
Mrs Dosunmu said: "It's so sad that his life has been taken away and it's feelings that can never go away and we will just have to learn to cope with it."
Ms Mallon also said police were looking into an attack on Michael's father to see if there were any links with his son's murder.
Mr Dosunmu suffered injuries to the back of his head three days before his son was killed and need 18 staples and stitches to the wound.
On Crimewatch, Michael's family described him as a "peace-loving boy", a regular churchgoer who was honest and not involved in any local gangs.
Michael was one of three south London teenagers shot dead in less than a fortnight in February.
James Andre Smartt-Ford, 16, of New Malden, south-west London, died after he was shot in front of hundreds of people at an ice rink in Streatham, south London.
A few days later Billy Cox, 15, was shot dead at his home near Clapham North Tube Station in south west London.
Five people - two men and three youths - have been arrested over Michael's murder and released on bail.