Anwar Hossain, 32, was convicted to six months' imprisonment under the Drug and Narcotics Act on Wednesday and sent to Nilphamari District Jail on the same day.

He was taken to Nilphamari Sadar Hospital at about 11:30 pm on Thursday, where doctors declared him dead.

The jail authorities said Anwar Hossain was taken to hospital with chest pain.

But his family members alleged that Anwar was tortured in jail.

Two under-trail prisoners -- Abul Hossain and Fazlul Haque--died in the jail last one year, according to sources in the jail.