A policeman was found dead in the washroom on the 18th floor of Nizam Palace on Thursday morning.
Mir Azhar Ali, in his mid-40s, was attached to the first battalion of Kolkata armed police and was deployed at the CBI office as a security guard, a senior police officer said.
Mir had gone to the washroom around 11am and did not return. Some of his colleagues went looking for him, the officer said.
After knocking on the door for a few minutes, Mir's colleagues broke open the washroom door and found him lying on the floor.
"We received an intimation around 11am that one of our colleagues on duty at the CBI office in Nizam Palace was lying unconscious inside a common washroom," a senior officer of the Bhowanipore police station said.
"A team from the police station brought him out of the washroom and took him to hospital where he was declared dead."
The police have started a case of unnatural death with the Bhowanipore police station.
Preliminary findings suggest the policeman, who hailed from Murshidabad, suffered a heart attack.
"We have informed Mir's family members about his death," the officer said.
The CBI office at Nizam Palace has been investigating irregularities in appointments in state-aided schools.