In Tenerife, the trial of a German began before a jury on Tuesday.
The man from Saxony-Anhalt is accused of murdering his wife, from whom he lived separately, and his ten-year-old son in April 2019.
He is also accused of attempted murder of his seven-year-old son.
He faces life imprisonment, which in Spain can only be reviewed after 25 years at the earliest.
Hans Christian Roessler
Political correspondent for the Iberian Peninsula and the Maghreb based in Madrid.
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According to the Efe news agency, his defense wants to claim a mental disorder. His Spanish lawyer announced that he would submit reports from "reputable" doctors and psychologists. His defense lawyers had criticized "pre-judgment" by the media. So far, the 44-year-old German, who owned a house in Adeje in the southwest of the island, had been silent about the crime and only reported a dispute with his 39-year-old wife.
His wife and two sons, who lived in Halle (Saale), had visited him in Tenerife.
The children allegedly knew nothing about the separation of their parents.
During an excursion, according to the judiciary, on April 23, 2019, he lured the family into a cave in the south of the island under a pretext.
Easter presents for the boys are hidden there, he is said to have said.
But then he began beating his wife and ten-year-old son with stones.
The seven-year-old escaped at the last minute.
He wandered through the solitude for hours until he came across people.
During a major search, the police found the two bodies, and the father was arrested at his home.