A £20,000 reward is being offered by detectives for help in finding those responsible for shooting a teenager dead in his bedroom.
Michael Dosunmu, 15, was shot at his home in south-east London on 6 February.
The Metropolitan Police believe the killing may have been a case of mistaken identity.
Detectives said the weapon, a Mac 10 machine-gun, was still on London's streets and could be used again.
The reward is for information leading to the arrest and prosecution of the person or persons responsible for the murder.
"I think there are at least 20 to 30 people who would have been present when it [the murder] was planned"
Six men have been arrested in connection with the shooting.
Five have been released on police bail and the sixth was released without charge.
Police believe the shooting of Michael, who had turned 15 two days before the shooting, 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.
During a recent Crimewatch appeal, Det Ch Insp Fiona Mallon said: "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.
Church-goer
"I am confident this will still be in circulation now."
Michael's older sister Shakira was also at home when two men forced their way into the house.
She gave her brother first aid and he was able to say a few words to her, but he died in hospital less than an hour later suffering multiple gun shot wounds.
He was a regular at the Celestial Church of Christ, in nearby Bird In Bush Road, and attended Archbishop Michael Ramsey Technology College in Camberwell, south-east London.
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.
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