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2020 Saudi rail deal

Recordings of the former King's alleged mistress Corinna zu Sayn-Wittgenstein-Sayn speaking with a former police chief were leaked to the press in mid-2018. Sayn-Wittgenstein claimed that Juan Carlos received kick-backs from commercial contracts in the Gulf States – particularly in the late-2000s construction of the €6.7 billion Haramain high-speed railway in Saudi Arabia – and maintained these proceeds in a bank account in Switzerland. She alleged that he purchased properties in Monaco under her name to circumvent the tax treatment of lawful residents, stating " because he me a lot, but because I reside in Monaco." She further claimed the head of the Spanish intelligence service warned her that her life, and those of her children, would be at risk if she spoke of their association. The allegations drew demands for Juan Carlos to be investigated for corruption in early June 2019.

Swiss authorities began investigating Juan Carlos in March 2020 in relation to a $100 million gift to Sayn-Wittgenstein in 2012. This donation was linked to alleged kick-back fees from Saudi Arabia. Sayn-Wittgenstein reportedly told the head Swiss prosecutor on 19 December 2018 that Juan Carlos had given her €65 million out of "gratitude and love", to guarantee her future and her children's, because "he still had hopes to win her back". A letter written by Juan Carlos to his Swiss lawyers in 2018 stated the gift was irrevocable, despite his having asked in 2014 for the return of the money. On 14 March 2020, The Telegraph reported that his son Felipe, King of Spain since 2014, appeared as second beneficiary (after Juan Carlos) of the Lucum Foundation, which had received a €65 million donation by King Abdullah of Saudi Arabia. On 15 March 2020, the Royal Household declared that Felipe VI would renounce any inheritance from his father. Additionally, the Household announced that the former king would lose his public stipend from the State's General Budget.

In June 2020, the public prosecutor's office of the Supreme Court of Spain agreed to investigate Juan Carlos's role as facilitator in Phase II of the high speed rail connecting Mecca and Medina, intending to determine the criminal relevance of events that took place after his abdication in June 2014. As King of Spain, Juan Carlos was immune from prosecution from 1975 to 2014 by sovereign immunity.

A further investigation by Swiss authorities was undertaken regarding €3.5 million paid from the Lucum Foundation to the Bahamas-based bank Pictet & Ciein for a society called Dolphin, which was controlled by the lawyer Dante Canónica, who also controlled Lucum.

In December 2021, the Swiss prosecutors dropped all cases due to the impossibility of proving any illegality.