A witness told the teenager's mother, who had suspected the crime scene on Monday in Tulse Hill in southern London, when she stabbed him.
The boy, locally referred to as John, was "bubbly" and "had manners," according to one of his friends.
The police were called to reports of a shootout in Greenleaf Close at 22:53.
Paramedics tried to save the teenager, but he was pronounced dead at 23.41.
Officers believe that the boy had suffered stab wounds rather than gunshot wounds, although the Met emphasized that the investigations are at an early stage.
To date, there have been 119 violent deaths in London this year, including two cases treated as self-defense.
One third of the remaining 118 cases (42) are victims aged 16 to 24 while 20 were adolescents.
The total is approaching the value of the entire last year after five fatal bites were suspected within a week.
In 2017, there were 118 murders in the capital, excluding the victims of the terrorist attacks on Westminster Bridge, London Bridge and Finsbury Park.
A witness after the murder of Tulse Hill said that the boy looked "like an angel".
Paulina Wedderburn saw the victim's emotional mother and father at the scene.
Ms. Wedderburn, who has been living in an apartment in Greenleaf Close for decades, laments the recent wave of violence and killings in the south of London and said that was not always the case.
She said, "What's going on? What's it? Why do they have to kill each other?
"When I grew up in the 70s, there was a fist fight, that was it, there were no knives.
"All you do is annoy families, and when you've seen Mom and Dad, it's heartbreaking."
She added that the victim's mother had previously been in the apartments to pick up her son, as she does not like to hang around here.
One of the boy's friends, who had dinner with him the night he was killed, said, "He was bubbly, he was the friendly guy, he was respectful, he had manners."
Two other girls said John was stabbed "five times" when he left a friend's house in his heart and stomach.
The friends said John, an aspiring rapper, attended school at the Park Campus in West Norwood.
Another friend who lives on the estate and does not want to be named, also believed he had heard a shot before he looked out the window and saw the excitement.
He said, "When you grow up here, you get used to it. You can tell the difference between a shot and a fireworks display. "
However, Metropolitan Police said in a statement: "At that time, there was no evidence that a firearm was fired. "
Detective Chief Inspector Richard Vandenbergh said, "We believe the victim was attacked by a number of suspects, and the altercation would undoubtedly have caught the attention of nearby people.
"Have you seen or heard anything, and if so, no matter how insignificant you think it may be, call – your information may prove crucial."
On Monday, London Mayor Sadiq Khan warned that it could take a generation to stop violent crime in the capital.
Victims of recent deaths not associated with the police include "revered" father Rocky Djelal, 38, who was fatally stabbed Wednesday in Southwark Park, Rotherhithe, southeast London.
The following day, 15-year-old Jay Hughes was stabbed to death in Bellingham, also in south-east London.
17-year-old Malcolm Mide-Madariola was fatally stabbed on Friday in front of Clapham South subway station in south London.
On Sunday, Ayodeji Habeeb Azeez, 22, from Dagenham, on Samos Road, Anerley, also in southern London, was fatally hit.