The pop star, 30, was struck by a cartridge from a pyrotechnic device, which knocked her unconscious at around 2 am local time. After paramedics took Ms. Sainz to a hospital near Madrid, she was pronounced dead.
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Diario Montañés reports that Sainz Garcia was performing during the final night of a four-day festival in Avila province, Las Berlanas, accompanied by the Super Hollywood Orchestra.
Approximately 1,000 people watched on as the incident occurred. The performance was immediately halted.
A doctor and nurse, believed to be in the audience, attended to the singer while they waited for paramedics to arrive.
According to Spanish newspaper El Norte de Castilla, Prones 1SL – the promoter of Super Hollywood Orchestra – called the incident ”unfortunate” and said the fault was likely down to a manufacturing error.
“From the direction of the company Prones 1SL agency that represents the Super Hollywood Orchestra and all our staff, musicians, assembly and artists in general, [we] express our most heartfelt condolences to the family and friends for the loss of our colleague Joana,” Prones 1SL said.
"She always showed exemplary behavior, both personal and artistic, in addition to great human quality. It's going to be for all of us very difficult to overcome her absence."
Footage circulating social media is said to show the moment the explosion happened.