Officers from the Guardia Civil police force told a news conference their investigation was now closed, after Francois Patrick Nogueira Gouveia was charged with the murder of his relatives.
The corpses of Marcos Campos Nogueira, his wife Janaina Santos Americo and their one- and four-year-old sons were discovered last month in plastic bags inside their home in Pioz near Madrid after a neighbour called authorities over the stench.
Marcos Campos Nogueira, his wife Janaina Santos Americo
The parents, both 30 and from Paraiba state in Brazil's northeast, had been dismembered and police said they suspected the attacker was Campos Nogueira's nephew, who returned to Brazil after the crime but subsequently flew back to Spain to turn himself in.
After "partially" admitting the crimes, the 19-year-old was charged Friday and jailed pending trial.
Nogueira Gouveia confessed that he had "an uncontrollable desire to kill, that he was unable to control it and that he was even aware of it" when he went to visit his relatives in Pioz on August 17th, said investigator Juan Jesus Reina.
"Patrick clearly knew what he was doing," so much so that he brought with him plastic bags, sticky tape and a knife, he added.
"He tells us all the time that he is not crazy," said Reina, who described him as a solitary, egocentric and narcissistic person.
On the day of the crime, his aunt let him in and he killed her in the kitchen with the knife, but says he does not remember the murder of the children, Reina said.
Hours later, his uncle returned home, and he killed him in the hallway.
After dismembering his aunt and uncle and putting the corpses in plastic bags, Nogueira Gouveia showered, rested a few hours before leaving the next morning, back to his "normal life," said Reina.
It was only a month later that police uncovered the crime scene.
After this was publicised, Nogueira Gouveia moved forward a flight to Brazil and left the country, before finally coming back of his own accord.