Police were called to Tardew House on Goswell Road at 19.11pm on Monday, 2 August.
Officers attended with London Ambulance Service paramedics and the London Air Ambulance. They found a 15-year-old boy with a stab wound to the back.
He was treated at the scene before being taken to an east London hospital where he remains in a critical condition.
Detective Chief Inspector Eliot Porritt said: “This attack on a teenage boy has, understandably, shocked the community.
“He is in a very serious condition in hospital and our thoughts are with his loved ones who, like us, are willing him to make a full recovery.
“We are determined to apprehend those responsible for this vicious attack.
“Although the offence took place quite late in the evening, I believe that there were people in the local area who would have seen or heard what happened.
“If you saw anything suspicious, heard anything out of the ordinary or have become aware of any other relevant information since this stabbing took place, we want to hear from you.
“Your call could make the difference.”
Two males, aged 15 and 16 were arrested on Thursday, 5 August on suspicion of attempted murder. They remain in custody at a north London police station.
Anyone who knows anything that could help the investigation is urged to call 101, providing the reference 7555/02AUG.
Information can also be provided to the independent charity Crimestoppers 100% anonymously by calling 0800 555 111 or visit crimestoppers_uk.org.