Night watchman Ian Riley and Inspector Barry Taylor, a father-of-two, were fatally shot at Sunny Bank Mills in Farsley on February 15, 1970.
Burglar Neil George Adamson, 31, of Primrose Hill, Pudsey, was arrested two days later when 60 police officers swooped on a terraced house in Colne, Lancashire. He was later sentenced to life imprisonment, with a recommended minimum term of 30 years.
Now relatives of both victims plan to join local people who remember the tragedy to hold a memorial service.
Paul Colbeck was eight years old when his uncle Ian was killed and recalls collections at his primary school in support of both families.
"When she lost her brother, my mother just went into meltdown," he said. "The family just weren't allowed to talk about it.
"Of course, with being unable to talk about things with my family, I was starved of all information.
"What I'm really looking forward to is meeting people who knew Ian or my mother. There's a lot of blanks to fill in for me really."
One of the driving forces behind the event is former police officer Mandy Cook, a niece of Insp Taylor.
"He was my mum's brother and she more or less brought him up," she said. "She never really got over it until the day she died. It was just heartbreaking."
Mandy, who was born the year after her uncle's death, has been to visit the mill on a number of occasions as well as Pudsey Police Station, where a memorial plaque hangs on the wall.
She also plans to take part in this year's Great North Run to raise funds in memory of both men for the Blue Lamp Foundation.
The charity, which provides financial support for the families of emergency service heroes, is close to her heart not only because of her uncle's death but also her own experience on the force.
Mandy, 48, said: "It's really weird because he was murdered on February 15 and I got accepted into the force on February 15, so it's kind of like I picked up where he left off."
After six-and-a-half years with Sussex Police, she was attacked and left with a spinal injury that prevented her from continuing to serve.
"Mum went absolutely mental when I said I was going into the police," Mandy said. "What happened to my uncle made me more determined.
"There were times when I was sent to things and it was always in the back of my mind, but you can't let yourself think those things. It was just a fluke that I happened to get assaulted that day."
The memorial service, led by the Rev Sue McWhinney, will be held at the mill in Town Street on February 15 at 11.30am.
There are also plans by the owners of the mill to install a memorial bench in the future.
Visit Mandy's fundraising page to pledge sponsorship for her Great North Run fundraiser.