Most of the passengers on a whale watching boat carrying 27 people that capsized off Vancouver Island were on the left side of the top deck when a wave struck the boat's right side, causing the vessel to tilt and roll over, investigators said Tuesday.

Five British nationals were killed, and the search continued for a missing Australian man. Twenty-one people were rescued after the Leviathan II capsized Sunday afternoon.

Marc Andre Poisson, Director of Marine Investigations for Canada's Transportation Safety Board, said Tuesday that having so many people on the left side of the boat "raised the center of gravity."

"We know that most passengers were on the top deck on the port side, that's the left side of the vessel. This would have raised the center of gravity, affecting the vessel's stability," Poisson said at a news conference in Tofino.

"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."

He said investigators have now interviewed the three crew members and some of the passengers. One life raft deployed and was used, he said. The full investigation is expected to take months.

The British Columbia Coroners Service identified the five victims, two of whom were British nationals living in Canada. They are David Thomas, 50, and his 18-year-old son Stephen, from Swindon in southern England; Katie Taylor, 29, of Whistler, British Columbia; Nigel Francis Hooker, 63, of Southampton, England, and Jack Slater, 76, of Toronto.

The Down Syndrome Association UK said in a statement that David Thomas was a "huge supporter" of the organization and "one of the driving forces behind the Swindon Down's Syndrome Group, where he was a trustee."

Stephen Thomas, who had Down Syndrome, "was a very talented young man and a gifted photographer," the association said in a statement.

"His love of photography started when he was eight years old. We were all delighted when Stephen's beautiful image Moraine Lake won the national My Perspective photographic competition last year," the association said.

"All of our thoughts and condolences are with the Thomas family at this terrible time," the group added.

Microsoft UK said David Thomas was an employee. "Our thoughts and deepest sympathies are with their family, friends and David's colleagues and we will be doing everything we can to support them," the company said.

David Thomas' wife, Julie, was rescued and is hospitalized with minor injuries.

Michele Slater Brown, of Milton, Ontario, said she was notified about her father's death "in the wee hours this morning," and called him "larger than life, a charmer, handsome, entrepreneur, engineer in the navy ... and a lovely dad."

Coroner Matt Brown said a preliminary investigation suggests those who died were on the top part of the boat and that they weren't wearing life-jackets because it's not required in the type of vessel they were in.

Investigators will review the weather, wreckage and the maintenance history of the 20-meter (65-feet) boat to determine why it capsized, said Poisson.

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 crew shot flares from the water which attracted the attention of local aboriginal fishermen who rushed to help rescue people, said Corene Inouye, the company's director of operations.

The boat capsized about eight nautical miles (14.7 kilometers) off Tofino, a popular destination for whale watchers that is at the very tip of a peninsula some 200 miles (320 kilometers) northwest of Victoria, the capital of British Columbia.

Fisherman Clarence Smith said he was reeling his lines for halibut when his friend saw a flare shoot in the sky. They raced to the scene in their small boat, 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 rescued two women who were clinging to each other, and finally got 10 people on the life raft onto their boat. Among those they picked up 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.

Jamie Bray, the owner of Jamie's Whaling Station, said the boat sank in an area it goes to every day. He said he is cooperating with investigators to determine what caused the boat to flip over. He said the boat has operated for 20 years "with an absolutely perfect safety record."

One of the company's boats also had a fatal accident in 1998. The vessel capsized during an excursion, leaving an operator and a passenger dead. Bray said that vessel was struck by a rogue wave but said the latest incident involved a much larger boat.

Australia's Department of Foreign Affairs and Trade on Tuesday confirmed that an Australian man was missing. 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 dead, AAP said.