An accountant went on a killing spree at a family party, allegedly drugging relatives then butchering 14 of them with a kitchen knife.
Hasnin Anwar Warekar, aged 35, slit their throats in the early hours of Sunday morning while they were sleeping, including seven children, his wife and his parents.
He then hanged himself, police in India said.
The only survivor of the massacre was his 21-year-old sister.
It took place in Thane, a city 20 miles from Mumbai, local media reported.
Police spokesman Gajanan Laxman Kabdule said: ‘The attacker, Hasnin Anwar Warekar, hung himself after slitting the throats of all other family members including his parents.
‘We still haven’t been able to speak with the attacker’s 21-year-old sister, the lone survivor of the attack, who is in deep trauma at a city hospital.’
Police are waiting for the results of tests which will confirm if the victims had been sedated.
Local media reported 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.’
TV footage showed crowds gathering as police removed the bodies, wrapped in sheets.
The Press Trust of India news agency said a possible motive was a property dispute, but police said commissioner Ashutosh Dumbre told he ABP news channel: ‘In our inquiry so far, no one has yet been able to give the reason for this.
‘He worked with a private company in Mumbai. There were no known financial troubles or disputes and now we are hoping that the lone survivor can tell us something about the trigger.’