The police recovered the boy's decomposing body from a park in Dwarka's Sector 18 area on August 1. His throat had been slit, they said.
During investigation, it was found that the minor, a resident of Badaun in Uttar Pradesh, was living in Punjab with his 42-year-old maternal uncle, who had come to Delhi before the body was found, the police said.
However, the man during the probe had tried to mislead investigators by claiming that he was not in Delhi and also had planned to falsely implicate his wife and her brother in the crime, they said.
Several raids were carried out in Uttar Pradesh's Badaun and Bareilly districts to nab him, but he remained untraceable, they said.
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On September 17, the police was informed that he would come to Kailashpuri, Sagarpur, to meet somebody. He was arrested there, the police said, adding that the knife used in the crime also has been seized.
The boy had come to Longowal to live with his uncle and his wife in 2016. During his stay, the accused started suspecting that his nephew and wife were in a relationship, and the boy was sent back, Additional Commissioner of Police (southwest) Gaurav Sharma said.
In order to implicate his wife and her brother, the man bought a SIM card in her name in Longowal, and called his nephew, Sharma said.
The man and the boy met at the Jakhal railway station in Haryana, and he brought him back to Delhi on the pretext of collecting his Aadhaar Card from his wife, he said.