A whale-watching boat carrying 27 people sank off the coast of British Columbia Sunday afternoon, killing five.

The Leviathan II, a 65’ passenger vessel with upper and lower viewing decks and capacity for 46 — the largest in the fleet belonging to Jamie’s Whaling Station & Adventure Centres — sunk just before 5 p.m. Sunday in waters near Tofino, British Columbia, on the west coast of Vancouver Island.

The five people killed were British nationals the UK Foreign Secretary confirmed on Monday. The Royal Canadian Mounted Police continued to search Monday for one person still missing, while 21 others were rescued from the water on Sunday.

It is not yet clear what caused the boat to sink — the weather was reportedly clear and sunny, with 9-10’ seas — not unusual conditions, according to another local tour operator.

“The sea was 3 to 4 metres, a fairly big sea, but not much wind or too unusual for the conditions we deal with on a regular basis out here,” John Forde of the Tofino Whale Centre told Canada's Global News.

The crew apparently was not able to send out a mayday call before the boat capsized, but emergency flares were deployed from the water. 

Jamie Bray, owner of Jamie’s Whaling Station, issued a statement Monday morning.

“It has been a tragic day. Our entire team is heartbroken over this incident and our hearts go out to the families, friends and loved ones of everyone involved. We are doing everything we can to assist our passengers and staff through this difficult time. We are cooperating with investigators to determine exactly what happened. In the meantime, we want to extend our most sincere thank you to the first responders, rescue personnel, and everyone from Tofino and the local First Nations communities who assisted with the response efforts.”

CBC News reported that another boat operated by Jamie’s Whaling Station, the Ocean Thunder, sank near Tofino in 1998, killing one passenger and the captain.

The incident is being investigated by multiple agencies, including the Transportation Safety Board of Canada, the Royal Canadian Mounted Police, and the Coroners Service of British Columbia.