TOFINO, British Columbia — Five British nationals died when a whale watching boat with 27 people on board sank off Vancouver Island, the British foreign minister said Monday.

One person was missing and the rest were rescued, some by members of the local aboriginal community who rushed to help.

The cause of the sinking remained a mystery. The boat made a mayday call late Sunday afternoon on a calm, clear and sunny day off Tofino, a popular destination for whale watchers, the Joint Rescue Coordination Centre said.

Jamie Bray, the owner of the company that operates the boat, said he is cooperating with investigators to determine what happened but they don’t know the cause. The crew used flares from the water, which attracted the attention of local aboriginal fishermen.

Bray said the boat sank in an area it goes to every day and said they are all traumatized and in disbelief. He said he’s had minimal contact with the crew.

“This vessel has operated for 20 years with an absolutely perfect safety record. This is something just totally out of the blue,” Bray said. “We just don’t understand and we won’t know the answers until the Transportation Safety Board finishes their investigations.”

British Foreign Secretary Philip Hammond confirmed in a statement that the five killed were U.K. nationals. He said consular officials in British Columbia were supporting family members of those who died.

“My thoughts are with the family and friends of all those affected by this terrible accident,” Hammond said.

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