Deputy Commissioner of Police (South Zone), V Satyanarayana said the boy had gone missing on June 28 and a missing persons case was filed by his parents at the Chandrayanagutta Police Station the next day.
Police formed two teams to probe the matter which also looked into the CCTV footages of the area concerned.
The victim was last seen walking towards a school building with the 17-year-old accused, the DCP said.
The accused lives in the victim's neighbourhood in Barkas locality and works at a pan shop.
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"Another shocking thing that came to light was that the teenager had allegedly sexually assaulted at least 15 minor boys over the past two years luring them with chocolates and biscuits," the senior police officer said.
The accused has also told the police that he himself was sexually assaulted by two persons about three years ago, the officer said.
The DCP said they are in the process of identifying the other victims and stressed on the need for creating awareness in the society over such crimes.