Suraj Baitha, also a worker of the mill, lost his life in Patna Medical College and Hospital (PMCH) late last night where he was being treated since April 10, East Champaran Superintendent of Police Jitendra Rana said.
Another union leader Naresh Shrivastava, who along with Baitha, had attempted self-immolation during a sit-in protest demanding non-payment of arrears and reopening of the sugar mill, had died in PMCH on April 11.
Chief Minister Nitish Kumar had ordered an inquiry into the incident.
Rana said that the manager was arrested from Lucknow on April 16 while the owner was absconding.
The SP said that relatives of the two victims have alleged that the owner and the manager of the mill had orchestrated their killing with the help of anti-socials.
The protesters had attacked a police team which had gone to the dharna site to take them to District Magistrate for talks and in the melee the two union leaders were seen burning. The fire was doused and the two badly burnt union leaders were rushed to PMCH on that day.