Dogan Ismail, 17, was stabbed Sunday at the same spot on the Aylesbury Estate in South London where Shaun Chambers, 21, was killed in September, The Sun reported.
Investigators said Orhan Ismail, 15, was robbed of his BlackBerry several days ago, recognizing some of his attackers as schoolmates. Orhan then got a call telling him he could get the phone back if he came to the Aylesbury Estate.
Orhan said he and his brother went to the agreed-on place and encountered a group of young men in a stairwell.
"He made to give back the phone, but instead brought a knife from behind his back and stabbed Dogan in the chest," Orhan said. "His last words were, 'Oh my God, you have stabbed me!' The guy just calmly walked away."
Police have detained a 15-year-old on suspicion of murder and his mother for allegedly perverting the course of justice. In an unusual move, they have identified another teenage suspect by name.
Ismail was the eighth teenager killed this year in London.