Two brothers with mental problems were beaten to death in their own house in Nong Khai province by an unknown group of assailants on Monday night, police said on Tuesday.
The Phone Pisai Police Station was alerted at 8am to the double murder at a house in Tambon Wang Luang.
The two brothers, 43 and 42 years old, were fatally hit on the head in their bedroom on the ground floor.
Their 73-year-old father told police that he had heard his sons howling in pain at about 8pm, but he did not dare to come down fearing for his own life.
When he checked in the morning, he found both his sons dead.
Pol Colonel Phuwit Siripanit, commander of the police station, said the two suffered from mental disorder following years of drug abuse since a young age.
Phuwit said police had learned from people in the neighbourhood that the brothers were accused by a man of stealing his smartphone and the man is suspected of having brought his friends to attack the two.