A 41-year-old father overwhelmed with family debt is believed to have killed his wife and two sons and then cut his own throat.
A call scam which cost his wife 1.7 million baht she had herself borrowed was thought to be the tipping point.
The tragedy happened at the family's house in Samut Prakan and was reported to local police about 1am on Monday.
Police found on the first floor the bodies of the 44-year-old wife and the couple's youngest son, a 9-year-old. Both had deep cuts to their neck and body.
On the second floor they found the second son, a 13-year-old, dead in front of a bedroom door. The husband, Sanit Dokmai, lay face up on the floor holding his bleeding neck, which had a deep cut across it. His wrist was also cut. Rescue workers gave him first aid and he was rushed to hospital.
The husband's work supervisor told police that Mr Sanit phoned her after midnight and told her he had killed his wife and two sons. She called his neighbour and rushed to the house while the neighbour called police.
She said Mr Sanit had earlier told her he was in danger of losing the house because he had guaranteed a car loan for another person who defaulted. He said he could clear the debt if he could get a small loan.
However, his wife had recently been tricked by a call scammer and borrowed about 1.7 million baht and transferred the money to the fraudster. Creditors were demanding repayment from his family every day.
The damage from the scam could well have been the final straw, leading to the tragedy, the supervisor said.
A colleague of Mr Sanit said the man had been worried about his family's debts.
Police said that last Friday the slain wife had filed a complaint. She said she had applied for a 100,000 baht loan using a phone loan app, and been tricked into paying more than one million baht for fees.
