A 35-year-old chartered accountant has murdered 14 members of his family, including seven children, with a butcher's knife.
Hasnin Anwar Warekar slit their throats during a family party then hanged himself, police in India said.
Warekar's 21-year-old sister was the only survivor of the attack in Thane, 20 miles from the city of Mumbai, in the early hours of Sunday morning.
She was taken to hospital after neighbors heard her screaming for help.
Police broke open the front door and found the bodies.
Thane police spokesman Gajanan Laxman Kabdule said Warekar's sister is in "deep trauma" and officers have not yet been able to speak to her.
He said the motive for the murders is "still sketchy".
Warekar attacked his family after lacing their food with sedatives, say local reports.
"Prima facie evidence suggests that the accused bolted all the doors of the house and murdered his family while they were asleep, with a knife that we found near his body," Ashutosh Dumbre, joint commissioner of Thane police, told reporters.
He could not confirm whether the victims had been sedated, saying investigators were awaiting medical results.
According to the Press Trust of India news agency, the killings were sparked by a property dispute.