Juan Carlos Oliva Bellido, 79 and his sister Araceli, 75, were found in a room in their flat after neighbours complained of a rotten stench permeating the apartment building in the eastern city.
Police entered the flat, where the pair had lived for decades, to discover that the two siblings had most likely been dead for at least a month.
They were found in a room filled with hanging car air fresheners in a scene detectives described as resembling that of the Hollywood blockbuster ‘Seven' starring Brad Pitt and Morgan Freeman.
The bodies were found bound in ropes then wrapped within sleeping bags filled with cat litter, a move police believe was designed to delay the smell of the decomposing corpses from reaching outside the walls.
A post-mortem conducted on Tuesday revealed that both had been strangled.
There was no sign of a break-in at the apartment and the doors had been locked.
One neighbour first reported the waft of something unpleasant in early May, but it wasn't until temperatures soared over the last week that stench became unbearable.
Neighbours told local newspaper Levante that the siblings "were thought to have been visiting their ancestral village".
One neighbour reported seeing an unknown man leaving the siblings' apartment in early May "who said that Juan Carlos and Araceli would be away for some time out of town visiting family".
Detectives are trying to trace the whereabouts of a man who had cared for the pair after the brother was convalescing from a recent operation but who has now disappeared.