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"At first glance, it does not look deliberate but we are still investigating that," said police on X, formerly Twitter.
The incident happened in the town or Nunspeet, around 70 kilometres (45 miles) east of Amsterdam.
"My thoughts are with the victims and their families," said Rob Jetten, who is poised to become the country's next prime minister.
"Huge gratitude and respect to the emergency services who are pulling out all the stops to provide the best possible care," said Jetten on X.