Dutch police on Sunday raised the death toll to six with seven other people hurt after a horse-and-trailer truck ran off a dike road, ploughing into revellers.
The accident happened around six pm (1600 GMT) on Saturday when the Spanish-registered truck left the road on the narrow Zuidzijdsedijk, 30 kilometres (18 miles) south of Rotterdam.
Police had first confirmed at least two dead after the vehicle drove into a community barbecue, which Dutch news reports said was being hosted by an ice-skating club.
"We now have six people killed and seven injured, one seriously after yesterday's accident," police spokeswoman Mirjam Boers told AFP.
"We are still investigating what exactly happened," she said.
Photographs from the scene identified the truck as belonging to the El Mosca company, based in Spain.
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Boers confirmed the driver had been arrested "and was not under the influence of alcohol," at the time of the accident.
Quoting witnesses, the Rijnmond regional broadcaster said the truck stopped for a moment at a T-junction on the dike, before taking off and ploughing into the revellers.
The incident has shocked the tight-knit community of the Nieuw-Beijerland region with police calling for more witnesses to help with the investigation.