"Going in at the source means major intervention work with youth workers, inoculating means going into schools, changing social norms, educating kids, teaching them what it is to be a man, teaching them how they don't need to carry knives", she added.
"I tried to save her at the time".
The teen, named locally as Tanisha, died after being "shot in the stomach" in Tottenham - just minutes before a 16-year-old boy was shot in Walthamstow, East London, in an unconnected attack.
Officers attended along with the London Ambulance Service and found the teenage girl suffering from a gun shot wound.
Describing the victim as a "good girl", she said: "She didn't deserve that".
A second boy, aged 15, is also being treated for stab injuries but his condition is unknown.
A 21-year-old man has been arrested on suspicion of murder and is now in custody at a west London police station.
There have been no arrests at this stage in relation to both incidents and they are not being treated as linked.
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The murder rate in London, United Kingdom is up a staggering 40% in just three years; topping New York City for the first time in recorded history and raising alarm bells throughout Britain over escalating stabbings and shootings across the capital.
A third incident on Sunday evening left a 16-year-old boy in hospital after he was stabbed near Bow in east London.
Last year alone, 80 people were killed by stabbing, and this year's pace of 29 so far is set to overtake that.
Police officers stand guard by flowers left at the scene of a knife attack in Russell Square on August 4, 2016 in London, England.
Britain's interior ministry said it was consulting on new laws to further restrict unsafe weapons, including banning online stores from delivering knives to residential addresses and making it an offence to possess certain weapons in public.
Other friends told how they tried to comfort Tanisha in her last moments, staying by her side after the fatal shooting.
New laws being considered include banning online stores from delivering knives to residential addresses and making it an offence to possess certain weapons in private.