Alan Hogg was shot while his wife Nhot Suddaen was bludgeoned to death with a hammer at their villa in the northern province of Phrae by men paid 50,000 baht ($1,540).
Their bodies were discovered Tuesday morning in graves two metres deep on their own land, Colonel Manas Kerdsukho, police commander in Phrae, told AFP.
Two suspects have been arrested and warrants issued for two other men including Varut Rattanasajjakit, Nhot's elder brother and the alleged mastermind accused of paying the killers.
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"The motive for the killings was a long-running internal family conflict, feuds and property," he said.
British media reports described Hogg, 64, as a millionaire from Edinburgh who built a lavish home with a swimming pool in the province not far from the Laos border.
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A Thai television station showed footage of an excavator digging up mounds of dirt in a search for the bodies.
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The British embassy in Thailand did not immediately respond to a request for comment but police said they had been notified.
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