TOFINO, British Columbia » Investigators sought to determine what caused a whale watching boat with 27 people on board to sink off Vancouver Island in seemingly calm weather, killing five people and leaving one person missing. A fisherman who helped in the rescue effort offered a clue Monday: a survivor told him that a sudden wave capsized the boat.

British Foreign Secretary Philip Hammond confirmed in a statement Monday 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.

Australia’s Department of Foreign Affairs and Trade on Tuesday said an Australian man was missing. DFAT said it was providing consular assistance to the man’s family.

Australian Associated Press reported that the 27 year-old Sydney man’s family said he was on the boat with his girlfriend and her family when it sank. His girlfriend’s father was among the five British citizens confirmed dead, AAP said.

The other 21 people on board the Leviathan II were rescued Sunday, some by members of the local aboriginal community who rushed to help.

Officials have not commented on what caused the boat to capsize. A senior employee of Jamie’s Whaling Station, the company operating the boat, said the vessel sank so quickly the crew didn’t have time to issue a mayday call. The boat capsized off Tofino, a popular destination for whale watchers.

The company’s director of operations, Corene Inouye, said the crew used flares from the water which attracted the attention of local aboriginal fishermen.

Fisherman Clarence Smith said he was reeling his lines for halibut when his friend saw a flare shoot in the sky. In their small boat, they raced to the scene, and saw people in life rafts, in the water, and on rocks. They first helped a man who was clinging to the side of the boat, taking eight minutes to get him on board. He was unresponsive, and tangled in a line.

Then they got two women who were clinging to each other, and finally got 10 people on a life raft onto their boat. Among those who they rescued were a pregnant woman and a woman with a broken leg.

"The lady was saying that a wave just capsized them. That’s why there weren’t any communications on the radio, no mayday," Smith said.

Government investigators arrived Monday afternoon in Tofino, a remote community of about 2,000 people at the very tip of a peninsula some 200 miles northwest of Victoria, the capital of British Columbia.

Marc Andre Poisson, Director of Marine Investigations for Canada’s Transportation Safety Board, said it’s too early to say what caused the boat to capsize it or what the contributing factors might be. He said the investigation could take months.

Poisson said they will review the weather, wreckage and maintenance history. He said they hadn’t yet spoken to the crew and passengers. He said the vessel has been towed to a nearby island but remains mostly submerged. He said the investigation could take months.

Jamie Bray, the owner of Jamie’s Whaling Station, said he is cooperating with investigators to determine what happened but said he didn’t know the cause.

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."

He said the passengers are not required to wear life jackets on larger ships like this.

It wasn’t the first fatal accident on the whale watching company’s record. In 1998 one of its vessels capsized during an excursion, sending all four people on board into the water. The operator and a passenger died. Bray said that vessel was struck by a rogue wave but said this incident involved a much larger boat.

The capsized boat, the 65-foot Leviathan II, was partially submerged 8 nautical miles west of Tofino.

The Royal Canadian Mounted Police sent an underwater recovery team to search for the missing person, with assistance from the Coast Guard and local search and rescue personnel.

Barbara McLintock, a coroner’s spokeswoman, said four men and one woman died and their ages ranged from 18 to 76. She said two of them were residents of Canada. Their names were not released.

Boats from the nearby Ahoushat First Nation arrived first on the scene, Robert Burridge said. Burridge estimates that every vessel that could be used in the village was in the water searching for missing people.

Canadian Prime Minister-designate Justin Trudeau thanked all those who participated in the rescue effort and offered his condolences to the victims and their families.

"I know firsthand of this coastal area’s natural beauty and the many people who visit here from all around the world," said Trudeau, who won Canada’s national election last week. "My thoughts and prayers are with the passengers, the crew, and their families at this most difficult time."

Sheila Simpson said she was strolling on the dock with her husband when rescue boats roared up carrying survivors from the distressed vessel as well as a man’s body covered by a blanket.

Simpson, who was in Tofino visiting a friend at the local hospital, said she tried to comfort some of the survivors as they stood on the dock awaiting transport to hospital or to their hotels.

"They were in absolute shock," said Simpson. "You could see it in their eyes."

Many of the survivors were taken to Tofino General Hospital and some were discharged Sunday night, said Valerie Wilson, a spokeswoman for the Vancouver Island Health Authority.

Tofino’s mayor commended residents for their quick aid in the rescue effort.

"Everybody’s heart is just breaking for what’s going on here and wanting to be as helpful as possible," Josie Osborne said in a telephone interview late Sunday.

Joe Martin, a member of the Tal-o-qui-aht tribe, was near the dock when rescue boats went out.

The ship was on the far side of Vargas Island in Clayoquot Sound, an area that Martin said can get really rough, but was not on Sunday.

"It wasn’t even blowing hard," he said. "This is the largest boat in Tofino and I was really surprised that it went down."

Hainsworth contributed from Vancouver. Associated Press writers Rob Gillies in Toronto, Rod McGuirk in Canberra, Australia, and Greg Katz in London contributed to this report.