The Marine Lines police have registered a case in the incident and are investigating the matter, police sources said.
Meanwhile, the family of constable Mohan Shankar More, who was posted in the chief minister's cabin, claimed that he had not returned home since Thursday.
Police said they are searching for him.
Earlier, forensic experts also visited the site as part of an inquiry being conducted by police to collect samples from the spot for probing the cause of the blaze.
One more body was recovered by fire brigade officials on Friday morning from the sixth floor of the building, taking the toll in the incident to three.
"The charred remains of a victim were found outside the office of Deputy Chief Minister Ajit Pawar on the sixth floor of Mantralaya at around 8 AM," fire brigade sources said.
Sixteen others sustained injuries in the raging fire in the secretariat. The injured persons have been admitted to various hospitals, including 11 at JJ and St George, 3 at G T and 2 at Nair hospital.
Some of them have suffered suffocation, while others have sustained minor injuries on hands and legs, J J Hospital dean Dr T P Lahane said.