The body of Pappu Kumar (25) was found floating in the water by other inmates who informed the authorities, the jail's Deputy Superintendent Santosh Pathak said.
When he was taken to hospital, doctors declared him as brought dead, he said adding that the pond was meant for fishery.
Kumar, who hailed from East Champaran district, was arrested by GRP from Siwan Junction two days ago for allegedly violating the Bihar Prohibition and Excise Act, 2016.
A probe was ordered into the incident while his family has been informed, Pathak said.
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