Most of the passengers on a whale-watching boat that sank off the coast of Canada were on the top deck on the left side of the vessel when a wave hit the right side, investigators have said.
Five Britons were killed after the boat overturned near Vancouver Island on Sunday.
A physical examination of the vessel is due to begin today, investigators for the Transportation Safety Board (TSB) of Canada said.
David Thomas, 50, and his 18-year-old son Stephen, from Swindon, Wiltshire, were among those who died when the Leviathan II capsized.
Nigel Hooker, 63, from Southampton, was also killed along with ex-pats Jack Slater, 76, who lived in Toronto, and Katie Taylor, 29, who lived in the ski resort of Whistler.
It has also emerged that none of those who died was wearing a life jacket although plenty were available.
Lifejackets not compulsory
Matt Brown, regional coroner for the Island Region of the British Columbia Coroners Service said current regulations do not require passengers to wear life jackets in that area or on that particular type of boat.
Marc Andre Poisson, director of marine investigations for the TSB told a news conference the position of most people on one side would have "raised the centre of gravity", and affected the boat's stability.
"We know that most passengers were on the top deck on the port side, that's the left side of the vessel," he said. "This would have raised the centre of gravity, affecting the vessel's stability.
"We also know that the sea conditions were such that a wave approached from the starboard quarter, that's the right of the vessel. We know that the vessel broached and then capsized."
The TSB has interviewed the three crew members and some of the survivors about what happened.
At least two hand flares and one parachute rocket were fired to raise the alarm, and one life raft was used, investigators confirmed.
The TSB said its investigators will try to recover any electronics from the boat to help determine its location when it capsized, and will look at the vessel's maintenance and inspection records.
No-one was wearing lifejackets
Speaking at a separate news conference Mr Brown said: "What we have found thus far is that none were wearing life jackets.
"As I understand through the current regulations that's not a requirement in the area that they were or on this vessel."
He added: "Our understanding at this time is that life jackets were on board. I believe that this vessel can occupy up to 50 individuals. There were 27 on board and there were life jackets available for all of them."
Mr Brown said the boat had two decks, adding: "The information we have is that they were at the top of the boat."
He said the top deck was open whereas the lower deck was enclosed by windows.
A 27-year-old man from Sydney remains missing, he said.
Mr Brown said the incident happened "very quickly" and the "chaotic environment" created "a very challenging rescue operation for many involved".
The boat, run by local tour firm Jamie's Whaling Station, got into difficulty eight miles from the small town of Tofino, around 150 miles west of Vancouver.
Following the incident the company's owner, Jamie Bray, said passengers on the boat were not required to wear life jackets.
"On larger vessels we're not required to have the passengers wear the life jackets. On smaller open boats they are," he said.
Local fisherman Clarence Smith said one survivor believed a wave had capsized the boat and a pregnant woman and another woman with a broken leg were among those rescued.
"The lady was saying that a wave just capsized them," Mr Smith said. "That's why there weren't any communications on the radio, no mayday."
Stephen's mother Julie was rescued from the stricken vessel, Leviathan II, along with 20 other people on board.
The Down's Syndrome Association paid tribute to Stephen, whom it described as "very talented young man" and a "gifted photographer", while his father was a "huge supporter" of the charity.
Stephen's brother, Paul Thomas, a 22-year-old University of Nottingham student, was flying out to Canada to be with his mother who is in hospital with minor injuries, the Swindon Advertiser reported.
Tragedy beyond belief
Mr Slater, who was originally from Salford but had lived in Canada for many years, was on board the boat with his wife Marjorie but she was rescued, the BBC reported.
One of his daughters described her father's death as a "tragedy beyond belief".
Writing on Facebook, Michele Slater Brown said: "Our hearts are broken today, our father was one of the people who lost their lives on the whale-watching tragedy in Tofino.
"Our dad was larger than life, a charmer, handsome, entrepreneur, engineer in the Navy, he was 76 years old, he was our dad, our lovely dad, I will miss him forever but I'm grateful for all the times I spent with him, I love you dad."
The boat began to take on water around two hours and 15 minutes after it took off on its whale-watching tour, the TSB said.
Wreck to be investigated
Investigators will now examine the wreckage of the vessel, its maintenance history and and consider the weather conditions at the time.
Mr Brown said it had not been decided if post-mortem examinations were required, and that the chief coroner would decide whether an inquest would be held.
He added that the findings of the coroners' investigation - which could take a year - will be made public through a report or an inquest.
Jamie's Whaling Station 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.