The victim, aged in his 60s, was found unconscious and not breathing when officers responded to a concern for safety report at a property in the Newcastle Road area of Kilkeel shortly before 1pm on Thursday.
The deceased’s wife, also aged in her 60s, was also found inside the property with serious head injuries.
PSNI Detective Chief Inspector Anthony Kelly said: “Medical treatment was provided to the woman at the scene – and she has since been taken to hospital for treatment to her injuries.
“She remains in a stable condition at this time.
“The man was sadly pronounced deceased at the scene.
“At approximately 2pm, officers detained and arrested a 25-year-old man on suspicion of murder and attempted murder. He remains in custody at this time, assisting officers with their enquiries.
“Enquires remain ongoing in order to determine exactly what has happened. However, at this time, we are not treating this as a firearms-related report.”
Newry, Mourne and Down Chief Inspector Lynne Corbett said: “We understand the Kilkeel and wider Mourne community will feel both shocked and saddened by what has happened today.
“Our thoughts are with the family as they struggle to come to terms with what has happened, and they have our full sympathy at this time.
“Please be reassured that our officers will be doing everything in their power to conduct a thorough investigation into the circumstances surrounding what has happened, and we would urge members of the public not to speculate.
“Cordons remain in place this evening as our enquiries continue. I am appealing to anyone who believes they may have information which could assist us with our investigation to contact us on 101 quoting reference 739 21/03/24.”
A major police operation is ongoing at the scene.
A previous PSNI statement said: “Cordons are in place as officers conduct a number of enquiries.”
Forensic officers have been seen entering the house.
The Northern Ireland Ambulance Service said it despatched a crew to the property.
One person was taken to the Royal Victoria Hospital in Belfast, a spokesperson said.
Local councillor Henry Reilly described the situation as “very very tragic”.
“The family are very well respected in the community,” he said.
“They are decent people.”
Another councillor, Glyn Hanna, said the incident has been described to him as a “major assault”. He said it was a “tragic situation”.
“Everyone is hoping this doesn’t turn into an even bigger tragedy,” he added.
“The family is very highly regarded and well thought of it in this tight knit community.
“It’s horrendous and I’m so, so sorry this has happened.
“Everyone is shocked and we are all praying that they [the couple] pull through”.
TUV leader Jim Allister tweeted: “Thoughts and prayers with all those affected and suffering because of the very sad events in Kilkeel.”
SDLP councillor Laura Devlin described the situation as “really worrying”.
“Some of the reports are very concerning but for now I await news from the police,” she said.
“I wouldn’t want to add to any speculation.
“At this stage the families of all involved are in my thoughts and prayers”.
The PSNI said it will provide an update in due course.