Just minutes later, a police officer and a friend of Jermaine's knocked at her door -- her little brother had been stabbed, they told her. They led Goupall to a spot just two streets away, where the 26-year-old saw blood seeping onto the pavement from a police tent. She pushed past onlookers to talk to the authorities, only to be told Jermaine was dead.
"I just collapsed and cried -- the trauma from that moment made everything black," Goupall told CNN at her home, in the south London borough of Croydon.
It's a scene that's become all too familiar in London, where knife crime is soaring, mostly among young men and boys, some of them incredibly young.
Among just two of the many recent incidents: a 14-year-old boy was left in a critical condition after being stabbed in the stomach by an 11-year-old in July, according to reports
. And in the east London borough of Newham, two boys, aged 16 and 13, were convicted of grievous bodily harm for stabbing another 13-year-old boy in September.
From June 2017 to June 2018, the latest period for which figures exist, the Office of National Statistics (ONS)
recorded nearly 40,000 knife or sharp instrument crime offenses. This was the highest in the eight years for which data is available, and a 12% increase over 12 months. Despite the rise, these specific crimes are relatively rare, when compared to the frequency of other violent attacks, according to the ONS.
And in February this year, London's monthly homicide rate briefly overtook New York's for the first time in history.
The ONS report says homicides in London rose 14% in the 12 months to June 2018. This year alone, the UK capital has seen 131 murders, police say.
Jermaine's death, in August last year, was a horrible case of mistaken identity, Goupall said, a view that has been backed by police
. Before his death, rival gangs had been wielding knives and threatening to kill each other in drill music videos
posted on YouTube, which have since been taken down. The gang members who killed Jermaine thought he was from a rival gang, according to Goupall.
The gang members, masked