Police in Spain believe missing Belfast man John George (aka John Hardy) was shot dead on a date between December 14 and December 17.
Their inquiries have now become a murder investigation, with Mr George’s family saying they know they are searching for his body.
On Saturday relatives visited the property where he was last seen, but the Spanish police arrived a short time later and declared it a crime scene.
The palatial apartment with private pool is in the exclusive Orihuela area of Alicante on the Costa Blanca.
Guardia Civil forensic officers removed a number of items for examination at the weekend.
Neighbours say fireworks for fiestas held around the time of the Belfast man’s killing may have masked the sound of gunshots.
Those living around the property were asked by police if they heard any shots during the three-day period.
Officers also collected CCTV and dashcam footage from people living in the surrounding area.
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The Belfast Telegraph spoke to one eyewitness.
They said around the time of Mr George’s disappearance there had been “frantic” activity at the property, with multiple cars arriving and leaving at speed and raised voices heard in the garden.
The neighbour said: “The place was all lit up with festive lights every night.
“Then, four days before Christmas, the place was in darkness and they haven’t been back since.”
Two young men, both from Northern Ireland, who had been staying at the apartment were questioned by police on Saturday but were not arrested.
Items were removed for examination and their passport numbers recorded.
The family say the PSNI needs to apply more pressure on its Spanish counterparts as they are concerned one of the main suspects is planning to flee to Thailand.
They have also ruled out the involvement of foreign crime gangs, and are now sure he was killed by people known to him.
Mr George, who had travelled to Spain for a pre-Christmas holiday, last made contact with his father Billy on December 14.
The neighbour who spoke to us added: “ This is a quiet area and there were a lot of people coming and going, lots of cars and lots of noise.
“I have been thinking about the family of the Irishman missing and we’ve tried to help the investigation any way we can.”
John George and sister Courtney George
The family were given false hope on Sunday when a female relative of one of the suspects in the case said she would send a location where his body could be found.
However, the tip-off was a map with a circle drawn around a 60 square mile area.
The missing man’s father said: “Our John was no angel. He was a boxer when he was younger, a really good one, and then went off the rails when he was 16.”
He added that he told his son “to come home”.
“We just need to find his body and we will not stop until we do,” he said.
“Those involved or covering up for them need to realise that.”