Naik's suicide note has pointed fingers a cabinet minister's kin responsible for his extreme step.
Sawant's assurance comes even as family members of Naik, who police believe died of self-inflicted gunshot wounds on Friday, have said that they would not claim the body, until police arrests the persons named in the suicide note for abetting his death.
"I have already instructed Special Branch to conduct a thorough investigation. I will get a thorough report by Monday. The preliminary report will be submitted to be by today (Saturday) evening," Sawant told reporters on Saturday morning.
Naik, 45, a realtor by profession, was pronounced dead on arrival at a local hospital on Friday morning. While police say that he died of gunshot wounds, the post mortem report which will formally identify the cause of death is expected to be pronounced on Saturday.
Before shooting himself Naik, a former sarpanch of Merces village in North Goa, had sent a WhatsApp message to friends and a social media group, in which he said that a cabinet minister's brother and a well known local businessman were blackmailing him as a result of which he was forced to end his life.
Naik contested the 2017 state assembly elections on an MGP ticket, but lost to the Congress party's Tony Fernandes.
Meanwhile, at a press conference at Naik's residence in Merces village near Panaji, the deceased's sister Sarita Govekar said, that the family had resolved not to claim the body from the mortuary, until police arrests the persons named in the suicide note.
"We want justice for my brother. Police has to arrest and investigate the persons named in the suicide note. Until that we will not claim the body from the morgue," Govekar said.
( With inputs from IANS )