The bodies were found at the family’s home in Phu Phiang district on Sunday night.
Police Sergeant 3rd Class Sitthisak Wangsan, 40, was found dead along with his wife Kanchana, 37, a nurse, and their twin boys, 7. He allegedly shot them in the head with his pistol.
Phu Phiang police investigator Lieutenant Anupan Sornsiri said the home’s four security cameras did not show anyone breaking into the house and there was no sign of a struggle.
Police suspect the incident took place on Friday as the boys were still in their school uniforms.
Anupan said police found Sitthisak was a good-natured family man who did not drink or smoke.
But his body would be examined to see if he was under the influence of toxins at the time of the killings. They will speak to relatives in the effort to find reasons for the tragedy.