Family and friends will gather in a small British Columbia town on Wednesday to remember one of the worst mass murders ever to occur on Canadian soil – a crime that remains unsolved 50 years later and one most Canadians have never heard of.

Canadian Pacific Flight 21 was en route from Vancouver to Whitehorse on July 8, 1965, when the pilot declared "mayday" over his radio. Minutes later, witnesses saw the plane plummet into the woods west of 100 Mile House, in the B.C. Interior.

All 52 people on board were killed, including six crew members and four children.

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An inquest determined the plane was brought down by an explosion "foreign to the aircraft and its systems," in the parlance of the day. Why and by whom remains a mystery.

"In those days, we weren't told much," recalled Didi Henderson, who was five when her father, Wallace Brooks Emo, was killed on Flight 21. "We had only been told that our dad died in a plane crash out west somewhere. We didn't know anything about the bomb or anything like that."

Mr. Emo was a geologist on his way to Whitehorse for work. He and his young family lived in Montreal.

"He was 33 when he died, and my mom was widowed with three kids under age seven," Ms. Henderson said.

"I remember very distinctly when we found out he died. … My sisters and I were all split up and sent to stay with different people for a couple of weeks. When we came back, my mom had moved us and we were back in the city, and it was life-altering without us having any understanding of what really happened."

What really happened was ultimately determined to be a bomb, set off in the rear washroom.

Every passenger was investigated, and the suspects narrowed down to a few possibilities, but the person responsible was never confirmed.

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"It's not important for me to know," Ms. Henderson said. "I'm not sure it could be solved now."

Murray Covello and his brother lost both parents. He will be in 100 Mile House to meet, for the first time, others whose lives were forever changed by the crash.

"My parents were on this aircraft en route to Vanderhoof to attend the funeral of my uncle, who had been killed in a construction accident in Prince George on the day before," Mr. Covello said. "There are few stories worse."

Just a child at the time, he relies on the accounts of family and friends to recall Dorothy and Tom Covello.

"I think about them every day of my life," Mr. Covello said.

In a Canadian Press story on front pages across Canada the following day, witnesses described the plane exploding in the air.

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William Wolfram, a logger working nearby, described the broken airliner and bodies strewn over a half-mile area as "spilled like an overripe fruit." Many of the victims were found still strapped in their seats.

Chuck Shaw-MacLaren, now 88, recalled a call from the local police, asking him to come out to the scene with rope and other supplies.

Over the next several days, Mr. Shaw-MacLaren and many of his friends and neighbours in the small town would help recover the dead from the crash site.

It was evening when they arrived that first night. Unable to start the search, they slept in their cars.

"It rained like mad, which when we started the search we were very thankful of because everything was washed clean. It wasn't as grisly as it could have been," Mr. Shaw-MacLaren said.

But it was a horrific scene, one still difficult for him to talk about five decades later.

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"We never spoke about it after. We tried to forget it," he said.

A plaque honouring the volunteers will be added to a cairn erected two years ago.

Having spent decades coming to terms with her grief, Ms. Henderson said it's important for the families to mark the occasion – and for the public to remember what happened.

"It's always called a tragedy and an accident, but it wasn't. It was a massacre," she said.

"Whoever did it, they willingly carried dynamite on board to kill themselves and 51 other people. So it's more than just a tragedy or an accident. It was kind of an act of terror, before we coined that term."