The injured have been shifted to Mumbai's J J Hospital, authorities said.
A R Nimbalkar (35), a bus conductor, had taken home a radio set found on a Mumbai-Ambejogai state transport bus yesterday. When he tried to replace the battery pack, it exploded.
Nimbalkar, his wife Usha (30), mother Kusum (53) and two-year-son old Kunal were seriously injured.
Superintendent of Police Dattatray Mandalik told PTI that forensic lab report was awaited, and officers of ATS as well as NIA had arrived to probe into the incident.
