While 69 people were injured, rescuers fear that some victims are still trapped under debris, they said. The deceased included 12 men, some of whom were construction workers, ten children and eight women.
A pregnant woman, identified as Shakila Imran Siddiqui, was among the deceased, Thane civic officials said. The remaining bodies are yet to be identified.
The Daighar police have registered a case against builders Salil and Khalil Jamadar under Section 304 (culpable homicide not amounting to murder) of IPC, police said, adding that efforts were on to trace them.
The seven-storey illegal under-construction structure, located in Shil Phata area between Mumbra and Dombivli townships, collapsed at around 6.30 PM yesterday.
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The injured have been admitted to various hospitals in Thane and some have been shifted to Mumbai's JJ and Sion Hospitals for better treatment, police said.
Relief and rescue operations are being carried out under the supervision of Civic Commissioner R A Rajeev and District Collector P Velarasu.
MNS legislator from Kalyan township of Thane, Ramesh Patil alleged that he had complained to the municipal commissioner last week that the building was unauthorised and unfit for occupation, but no action was taken.
There were around 35 families residing in the building, they said.