"Prima facie it is suspected to be a conspiracy. Four persons mostly government employees have been detained and they are being interrogated," Home Secretary Basudeb Banerjee told reporters here.
He said that fingerprint experts were also being requisitioned as part of the investigation.
An alarm was raised by a police constable yesterday after finding files and books doused with kerosene lying in the Home Department's publication office on the G-block of the building's top floor.
The state secretariat had moved out across the river to 'Nabanna', a 15 story building in neighbouring Howrah district following renovation of the Writers' Buildings.