SYDNEY — It was meant to be a day of celebration for the children of Hillcrest Primary School in the Australian island state of Tasmania.

       Instead, the final day of the school year was marred by tragedy: Four children were killed and at least five others were seriously injured when a bouncy castle was blown into the air by strong winds. As the castle took flight, with the students and a number of inflatable plastic balls inside, nine children tumbled more than 30 feet to the ground below, Tasmanian police said.

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       The four dead — two boys and two girls — were sixth graders. They had been due to begin high school in the new year. (Australian schools follow the calendar year.)

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       “On a day when these children were meant to be celebrating ... we’re all mourning their loss,” Tasmanian Police Commissioner Darren Hine told reporters. “Our hearts are breaking for the families and the loved ones, schoolmates, teachers of these young people who were taken too soon.”

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       It wasn’t immediately clear whether the castle was tied down at the time of the incident, or how many children were on the castle when it took off. An investigation is underway.

       Pictures posted on social media by an Australian Broadcasting Corp. reporter showed part of the deflated castle dangling from a nearby tree. A blue plastic tarpaulin cordoned off an area where the children were likely to have landed. In another photo, two police officers huddled on the grass in front of a deserted playground, comforting each other.

       “This is a very tragic event and our thoughts are with the families and the wider school community, and also our first responders,” Commander Debbie Williams told reporters. “There is no doubt that this has been a very confronting and distressing scene.”

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       Parents rushed to the school, uncertain whether their children were among the injured, the ABC reported. In a statement posted on Facebook, the school asked parents to collect their children “as a matter of urgency” and said it would close for the rest of the day.

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       It’s not the first time bouncy castles have been sent flying by strong winds. In the United States, four children were reportedly injured in Mesa, Ariz., in May when a gust of wind picked up a bouncy castle along with several children who were playing inside. The structure flew several feet and then collapsed, local media reported at the time. The children, aged 5 to 11, were taken to a nearby hospital after some reportedly suffered severe injuries.

       In 2017, a 6-year-old girl was killed in Spain after an inflatable castle went soaring into the sky with seven children inside. A similar incident led to the death of a 7-year-old girl in Britain in 2016.

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       Australian Prime Minister Scott Morrison said the Tasmanian incident was “unthinkably heartbreaking.”

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       “Young children on a fun day out, together with their families and it turns to such horrific tragedy, at this time of year, it just breaks your heart,” he said.

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