A 35-year-old Indian man killed 14 members of his own family, including seven children and his parents, before killing himself, police say.
Reports suggest Hasnin Warekar laced his family’s food with sedatives before slitting their throats.
Neighbours from the home in Thane, near Mumbai, were alerted to the attack by the screaming of the man’s sister, who survived the attack.
Police say it is still unclear why Warekar carried out the attack.
Authorities are yet to question Warekar’s sister, Thane police spokesman Gajanan Laxman Kabdule told AFP, as she was “in deep trauma” at a city hospital.
“He used a big knife. He killed his parents, his sisters and his sisters’ children. He slit their throats” Mr Kabdule said.
The Press Trust of India news agency reported that the youngest victim was his three-month-old daughter, and the oldest his 55-year-old father.
He also killed six of his sisters’ children, the report said.
The attack began in the early hours of Sunday morning, after everyone had gone to bed, he said.
Television footage showed bodies being carried out of the home, covered in sheets.
The family had gathered for dinner in Thane, about 32km (20 miles) from Mumbai, on Saturday evening.
Thane police commissioner Ashutosh Dumbre said the accused “bolted all the doors of the house and murdered his family while they were asleep”, using a knife that was found near his body.
Local media reports said the family members were drugged, but that has not been confirmed by police.
The victims’ bodies were discovered on Sunday morning in the man’s home.
Warekar is believed to have worked as an accountant in Mumbai.