Five people died after their boat capsized off the coast of Kaikōura, New Zealand. The incident took place on Saturday, September 10. Six people survived the incident and were rescued by the Coast Guard. Authorities suspect that a collision with a whale capsized the 28-foot fishing boat the people were on.

“It always plays on your mind that it could happen,” Kaikōura Mayor Craig Mackle told The Associated Press. Mackle said that the water was dead calm at the time of the incident. He suspects that a whale surfaced from beneath the boat. The area where the incident occurred is a popular whale-watching destination known to have a large number of sperm and humpback whales. It’s not clear which species of whale was involved in the accident. Mackle added that he’d never heard of a whale-boat collision occurring until now.

“This is an unprecedented event that has occurred,” Kaikōura Police Sergeant Matt Boyce told Stuff. “This has been a tragic event and the police are providing support to those involved at this very difficult time.” According to news reports, the boat involved was typically used as a fishing charter. When the incident took place, the captain was reportedly taking a group of bird watchers from a local birding group on an excursion. Police helicopters immediately responded to the situation. Divers helped recover the bodies of the deceased, which were all inside of the boat.

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Maritime New Zealand has assigned several investigators to undertake a full investigation of the tragic event once the recovery operations are complete. Authorities have not released the identities of the victims.