Man was being questioned on suspicion of murder on Monday night after the body of a man in his late sixties, believed to be his step-father, was found in a car in Lisburn.
The victim, named locally as Gerry Mulligan, was discovered in a car outside his home on Limehurst Way, off Low Road in the Co Antrim city, shortly after 4.20pm on Monday.
It is believed Mr Mulligan had been shot. Police said a post mortem examination will be held to determine the cause of death.
The father-of-four is believed to have lived on his own in the house.
Ulster Unionist MLA Jenny Palmer said the area was stunned by the death.
“It’s just tragic for his family,” she said. “My sympathies go out to them.”
“I am sure that in the fullness of time the PSNI will find out all of the circumstances. At the moment, we do know that a man his lost his life.”
SDLP Lisburn councillor Pat Catney said Limehurst Way was a “very quiet, settled, mixed area”.
“It does seem to have been a domestic incident,” he said. “But whatever happened, families are grieving.”
Alliance MLA Trevor Lunn has expressed his shock over the incident.
“This is a sad incident,” Lagan Valley MLA Mr Lunn said.
“Whatever the circumstances surrounding this death, no-one deserves to die in this way. The local community are shocked at this news.
“My thoughts and prayers go out to the family and loved ones of the deceased. If anyone has information on this incident, I would urge them to contact the police immediately..”
Detective Chief Inspector Peter Montgomery said: “Police are investigating the circumstances surrounding the death of man whose body was discovered in a car in the Limehurst Way area of Lisburn this afternoon, Monday September 26.”
He appealed to anyone with information, or who was in Limehurst Way on Tuesday between 9am and 5pm to contact police.
A police spokesman said officers do not believe there was any paramilitary involvement in the death.