"After Tabrez was thrashed, he was given water mixed with 'dhatura' (a form of a poisonous leaf)," said Mohammad Masroor, uncle of Tabrez.
"A police chargesheet should be filed immediately and the accused punished", he added.
Eleven people, including the main accused, have been arrested and two police personnel have been suspended in the case.
The Congress, Jharkhand Mukti Morcha (JMM), Rashtriya Janata Dal (RJD) and Left parties staged a sit-in on Wednesday at the Governor House demanding a CBI probe into the incident.
The Minority Commission chairman Mohammad Kamal Khan visited the village on Tuesday.
Tabrez Ansari (22), died in a hospital on Sunday, days after he was brutally thrashed by a mob at Dhatkidih village in Seraikela district on suspicion of stealing a bike on June 17. According to the police, a stolen motorcycle and some other things were also recovered from him.
The incident came to light after a video went viral in which the accused was seen beating Ansari.
Ansari's wife Shaista Parveen lodged a complaint with the Seraikela police station, alleging he was caught by some people while he was on his way back on a bike from Jamshedpur. He was tied to a tree and beaten up brutally and also forced to chant 'Jai Sri Ram'.
After the thrashing, the mob handed him to the police, which took him to a police post. However, he had to be admitted to a hospital after his condition worsened and he died on Sunday.
Ansari's wife has questioned the role of the police in her husband's death, alleging he was not given timely medical treatment.
( With inputs from IANS )