Hospital Superintendent Nirmal Bera said the undertrial, Upen Burman, who was in judicial custody in connection with the Greater Coochbehar Rail Roko protests in February, had been brought from Cooch Behar jail for an operation on his leg.
The surgery was performed yesterday but Burman could not be kept in the general bed, as is the practice, since there were no police personnel. So he was kept at the prisoners' room along with another undertrial, Nikhil Roy, who had undergone an operation earlier, Bera said.
Superintendent of Police Manoj Verma and Sub-divisional police officer Shyam Thapa rushed to the spot and broke open the door of the room.
Roy, who was under trial in connection with murder and lodged in a jail here, locked himself in the bathroom and was brought out after lot of persuasion.
He told the police officers that he had smashed Burman's head with a metallic saline stand after a heated exchange of words in the night, the police said.