The couple's bodies were found by police behind a duck pond at their luxury mansion in northern Thailand.
The couple have lived in their big compound, comprising a two-storey house and a cattle-yard, for years.
Over the weekend, police Major-General Sanpat Praputsra told reporters that investigators were "certain" that the disappearances were connected to a "long-standing conflict within the family" and that they were "confident" the culprit would be brought to justice. The hole was re-excavated, but police did not find any signs of one or more body having been buried there.
Edinburgh-born Hogg, a retired engineer, and his wife had been reported missing by friends last week, Sky News reports, after they failed to turn up to a pre-arranged meeting. He was charged with theft of the vehicle but was later released on 100,000 baht bail raised by relatives.
Major General Praputsra said that Alan's brother-in-law denied any involvement in their disappearance, saying he was only there to take the vehicle, and police did not have enough evidence to charge him.
Police were again searching for him. Everything would be clear in a few days, he added.
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'The brother-in-law had problems with money and there were family issues'.
Investigators learned the truck had been bought from a Thai couple by a Lao man. The sellers and buyer agreed to meet in Sri Samrong district in Sukhothai to close the deal last Friday (Sept 21).
The Lao man then drove it to Ayutthaya on business.
Police had previously interviewed Satchakit, who was seen on CCTV footage entering the couple's property after they were reported missing and leaving in their white pick-up truck.
"The three men conspired in the murder of the couple and our investigation is ongoing to determine exactly who did what", deputy police spokesman Colonel Kissana Phatanacharoen told Reuters. Police are hunting down the alleged mastermind, the wife's elder brother.
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