The arrested persons include a distant relative of the boy, they said.
Yash alias Aryan, son of medicine trader Prakash Gupta in Baikunthpur village, was abducted last evening while he was on his way to his father's shop near his residence.
Later, his abductors called up Gupta on his mobile phone and demanded a ransom of Rs 20 lakh, police said, adding that they also threated Gupta of dire consequences if he approached the police.
On knowing about the case, police launched a hunt to trace the abductors on the basis of their mobile call location.
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However, kidnappers apparently came to know about the police's involvement late last night and called up Gupta. They doubled the ransom amount to Rs 40 lakh for Yash's release, police said.
But, Yash's body was found in Janakhai forest area in the morning, police said.
The residents of the area staged a protest in front of the local police station, demanding immediate arrest of the accused.
Police said the mastermind of the crime, Gaurav Vishwakarma, is missing.