A boat which sank during a whale watching trip, killing five Britons including a teenager and a 76-year-old man, was capsized by a wave, a survivor has reportedly said.
Investigators from the Transportation Safety Board of Canada (TSB) have begun their enquiry into what happened to the vessel, which turned over near Vancouver Island off the west coast of Canada on Sunday, killing a woman and four men.
Twenty-one people were rescued from the stricken vessel, Leviathan II, which was carrying 24 passengers and three crew.
A 27-year-old man from Sydney is missing, Australia's Department of Foreign Affairs and Trade said.
The missing man's girlfriend's father was among the dead, the Australian Associated Press reported.
The victims included an 18-year-old man and another man aged 76, Barbara McLintock, from the British Columbia coroner's office said.
She confirmed that the British victims included three male tourists and two British nationals who lived in Canada - one woman from British Columbia and a man from Ontario.
The boat, run by Jamie's Whaling Station, a local tour company, got into difficulty eight miles from the small town of Tofino, around 150 miles west of Vancouver.
The company's owner Jamie Bray said people were "traumatised" and in "disbelief" at what had happened.
He said: "This vessel has operated for 20 years with an absolutely perfect safety record. This is something just totally out of the blue.
"We just don't understand and we won't know the answers until the Transportation Safety Board finishes their investigations."
A fisherman whose friend saw a flare in the sky said he rushed to the scene and described seeing people in life rafts, in the water and on the rocks.
Clarence Smith said they were able to help rescue a number of people, including a pregnant woman and a woman with a broken leg.
Mr Smith said: "The lady was saying that a wave just capsized them. That's why there weren't any communications on the radio, no mayday."
Corene Inouye, director of operations for Jamie's Whaling Station said the incident appeared to have happened so quickly that no mayday call was sent, but flares were lit.
"To the best of our knowledge there was no distress call. From what we know at this stage it appears that the incident happened so quickly, the crew didn't have an opportunity to send out a mayday.
"We have learnt that the crew was able to access emergency flares that are a part of the safety equipment on board the boat, and deployed them from the water.
"Local Ahousaht First Nations fishermen were the first to see this, and rushed to the scene to come to the assistance of our passengers and crew."
The TSB said the boat began to take on water around two hours and fifteen minutes after it took off on its whale watching tour.
During its investigations the team will interview crew members and passengers, examine the wreckage of the vessel, its maintenance history and how the equipment was operated, and look at meteorological conditions.
The TSB said: "Investigations are complex and we take the time we need to complete a thorough investigation. However, should the investigation team uncover safety deficiencies that present an immediate risk, the board will communicate them without delay so they may be addressed quickly."
On its website, Jamie's Whaling Station says it offers tours in Tofino and Ucluelet on "state of the art high-speed, exhilarating zodiac-style vessels" for those who crave adventure, or, it adds, who wish to "relax aboard the west coast's largest cruiser style whale watching vessels, the 65ft Leviathan II in Tofino or Lady Selkirk in Ucluelet".
The boats can hold up to 47 passengers who can "watch whales in their natural environment".
Following the incident Mr Bray said passengers on boats with enclosed compartments do not wear life jackets as it could make getting out of the vessel during a sinking difficult.
"On larger vessels we're not required to have the passengers wear the life jackets. On smaller open boats they are," he said.
Video footage online showed the boat bobbing vertically in the water, with what appears to be a large section of it submerged below the waves after it went down at around 4pm local time (11pm GMT) on Sunday.
The company suffered a previous fatal accident, with a boat becoming swamped and rolling to an angle in 1998, killing the captain and a tourist, and an incident two years earlier when a captain suffered head injuries, but survived, after falling asleep and running a boat aground.
Foreign Secretary Philip Hammond said his thoughts are with the family and friends of those affected by Sunday's accident.
The Canadian prime minister-designate Justin Trudeau said he was "shocked and saddened" by the deaths, adding: "We thank all those, including our search and rescue officials, who responded swiftly with courage and professionalism.
"I know first-hand of this coastal area's natural beauty and the many people who visit here from all around the world.
"My thoughts and prayers are with the passengers, the crew, and their families at this most difficult time. We will continue to offer them support in the days ahead."
Lieutenant Paul Trenholm, from the Joint Rescue Coordination Centre in British Columbia, said people from nearby indigenous First Nation villages had been first on the scene.
He said: "Without the assistance of the First Nation community this could have been much worse."
Locals held a dinner on Monday evening at the Tofino community hall to remember those affected by the tragedy.