A 16-year-old boy is critically ill after being stabbed outside East Croydon station on Friday.
The teenager was attacked by a suspect in his teens near to the junction of George Street and College Road at around 6pm.
Police say they are hoping to trace a man and a woman who tried to stop the attack and help the victim. Both were carrying shopping.
The attacker is described as a black teenager, 6ft of slim build with short dark hair.
Detective Sergeant James Hallums from Croydon CID said: "At the time of the attack, the victim was wearing a white, patterned, hooded jacket and jeans and we understand whilst there appears to be one main suspect there were others nearby who may have been connected to the attack."
Police said today the victim, who comes from Croydon, was still in a critical condition in hospital.
Anyone with information should call Croydon CID on 020 8649 0258.
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