Police in India say an accountant stabbed 14 members of his own family to death with a butcher's knife, including seven children.
Hasnin Anwar Warekar drugged his wife, children and parents and then slit their throats at a party on Saturday evening before hanging himself, according to reports.
The only person to survive the attack was his 21-year-old sister.
NATIONAL 24/7 CRISIS SERVICES
Neighbours heard Warekar's sister screaming for help some time after midnight, following a family gathering on Saturday evening.
Indian media have reported that Warekar either laced the evening meal with a sedative or waited until his family had gone to bed before bolting the doors of the house and beginning his assault.
Thane Police spokesperson Gajanan Laxman Kabdule told AFP news agency that investigators were awaiting forensic analysis to determine whether the victims had been drugged.
He added that details of the attack were "still sketchy."
Ashutosh Dumbre, joint commissioner of Thane police, said: "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."
The Indian media are reporting that a property dispute sparked the killings, but Mr Dumbre said that "there were no known financial troubles or disputes" that could have been the "trigger" for such an "extreme step." His remarks came in an interview with the ABP news channel.
It is hoped that the sole survivor of the attack will be able to shed some light on the events of Saturday evening, but Mr Kabdule said: "We still haven't been able to speak with the attacker's sister... who is in deep trauma at a city hospital."
Thane is a city of around 2 million people, while with around 20 million citizens Bombay is the most populous metropolitan area in India.
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