Police are appealing for information following the incident in Newsham Park, Liverpool, on August 31. Lloyd Medland and partner Kelly Ainsworth were walking their two dogs when one of their pets, Flossy, was attacked by a Bullmastiff-type.
They rushed the three-year-old Bichon Frise to the vets but she died of her injuries shortly after.
Merseyside Police urged anyone with information to contact them.
Inspector Phil Ryan said: “This was an appalling incident leaving a dog sadly dead.
“I want to make it clear that we will not tolerate irresponsible dog ownership.
“It is a criminal offence to allow a dog to be dangerously out of control in a public place and dog owners have a responsibility to ensure their dog is on a lead and under control.
“I would also ask anyone with information about this incident or dangerous dogs in their area to contact us so that we can take action.”
Mr Medland and Ms Ainsworth were walking Flossy and their other dog, an eight-year-old Staffordshire Bull Terrier called Leah, when the incident took place.
The couple had both of their pets on leads when Mr Medland heard a scream.
He told the Liverpool Echo: “I was stood with my back to my partner who had hold of the two dogs and I just heard screaming from behind me.
“I ran over and I saw my partner on the floor, my other dog was now loose running around.
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He added: “Our dog was in a bad way, it’s something I’ll never ever forget.
“She was screaming, I felt sick and scared because the dog was an absolute animal.
“I’ve got hold of our other dog but I’m also watching my dog getting killed. I was helpless, absolutely helpless.
“I felt helpless and I couldn’t react because if I let go of our other dog and we manage to get the big dog off it might go for that dog then.”
Flossie died shortly after arriving at the vets.
Mr Medland and Ms Ainsworth later contacted the police.