They found a 15-year-old boy with injuries but despite the efforts of the emergency services, he was pronounced dead at the scene.
The teenager has been formally identified as Kayjon Lubin, of Newham.
A post-mortem examination has established cause of death as a single stab wound to the chest.
The investigation by the Met’s Specialist Crime Command (Homicide) continues at pace.
Detective Chief Inspector Mark Wrigley said: “We are providing specialist support to Kayjon’s family as they go through the most unimaginable of times, and we are doing everything we can to identify who may be responsible for his murder.
“There were lots of people in the area at the time Kayjon was attacked and I would appeal for anyone who hasn’t yet come forward, to do so.
“Anyone who saw any activity near the Nisa store in Pier Parade at around 18.30hrs is asked to contact my team of detectives. No matter how insignificant the information may seem, we want to hear from you without delay.”
A 17-year-old male arrested on suspicion of murder and a 28-year-old man arrested on suspicion of attempted murder (both on the morning of Sunday, 13 December) remain in custody.
A 25-year-old man arrested on Saturday, 12 December has been released and no further action will be taken against him.
Anyone with information is urged to detectives in the incident room on 020 8345 3715, or contact the independent charity Crimestoppers, 100 per cent anonymously, on 0800 555 111.