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Police said the unnamed brother was closest to the victim and his family and was known to have complete knowledge of the victim's businesses.

The brother could also tell police which valuables had not yet been returned by the two suspects, both taxi drivers.

Pol Colonel Pornsak Surasit, the chief investigator, said one suspect had provided some useful information but that the other insisted he was innocent and did not cooperate. Police were producing further evidence to verify the information given.

Police are looking into other motives besides theft after safe-deposit boxes and most valuables were left untouched.

Witnesses reported hearing a break-in many hours before the estimated time of the killings, a sign that the killers waited for all of the victims.

The housemaid also reportedly let in two strangers who knocked on the door not long before the killings took place, suggesting the men might have been familiar with the house or the maid personally.