The move adds to pressure on current Socialist Prime Minister Pedro Sanchez, already weakened by corruption investigations involving family members and senior political allies.
Zapatero, who served as prime minister from 2004 to 2011, was not originally closely aligned with Sanchez but has in recent years become one of his top defenders.
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He had been summoned to testify on June 2, and police had searched his offices and three unnamed companies, Spain’s top criminal court said in a statement.
The investigation centres on a €53-million (US$62 million) emergency loan granted in March 2021 to Plus Ultra, a minor Spanish airline that operated flights to Ecuador, Peru and Venezuela with a small fleet of Airbus A340 aircraft.
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