The incident was reported from Sarita Vihar last night and it came to light when Daljit Singh (80) informed police that he had found his grandson Karandeep Arora (18) poisoned in their house, a senior police official said.
Police rushed Karandeep to a nearby hospital where doctors declared him brought dead.
Later, police found the bodies of Karandeep's parents Saminder Arora and Nimmi Arora from a railway track near their house.
A suicide note was recovered from them in which the couple claimed that they were ending their lives due to the heavy loss they encountered in business in Kondli.
A case has been registered and bodies have been sent for postmortem.
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