Police appeal for camera footage of brutal killing
The police officer leading the hunt for Mr Horner's killer and murder gang behind his death has appealed for phone and dashboard camera footage of the attack.
Movements of a red Ford Mondeo car used by the killer are also being tracked, and police are trawling CCTV footage taken at Sainsbury's and the surrounding area for clues.
Detective Superintendent Richard Campbell added in a scathing attack that Mr Horner's child faces being scarred for life by witnessing the shooting.
In a briefing at Bangor police station, he hit out: "It's a hugely barbaric act that will live long in this young boy's memory."
He said the attack had a "huge degree of recklessness" with one stray bullet found to have hit another car at the Sainsbury's car park during the shooting, in which it is believed that a total of six bullets were fired. DS Campbell admitted Mr Horner - whose full name was James Colin Horner - was "known to police" and said "one line of investigation was ongoing loyalists tensions in Carrickfergus".
But he added questions about Mr Horner's paramilitary involvement should not "take away from the fact a man with his three-year-old son was gunned down in a supermarket car park".
Outlining what police knew of the final few hours of Mr Horner's life, the Serious Crime Branch detective said the victim had been living with his partner and son in the Rathgael area of Bangor "for some time".
He left the house at around 1.30pm in his black Nissan Pulsar car registration XFZ 4706 before arriving at Sainsbury's around 2.30pm - meaning his whereabouts are unknown for around an hour.
Police want to know what happened in those 60 minutes before Mr Horner arrived at the car park where he was shot.
DS Campbell said although officers were scouring CCTV footage from around the car park and inside the store, they wanted the public's help to fill in the blanks of the murdered man's final movements - and urged anyone with mobile phone or dashboard camera footage of the attack or the victim's movements to hand it in to officers.
Detectives believe the gunman made his escape with at least one other accomplice in a red Ford Mondeo, that was later found burned out.
DS Campbell added the getaway car was missing its rear driver's side hub cap and had the fake number plate GKZ 7996.
It is thought to have driven off in the direction of Six Road Ends before it was found burnt out on Kerrs Road between Newtownards and Bangor.
Mr Campbell said the vehicle may have been following Mr Horner in the run up to the shooting.
He added: "There was the gunman and at least one other associate. So they must have had another vehicle on the Kerrs Road.
"There were a number of people we are yet to speak to and urge them to come forward and speak to police in Bangor.
"I am conscious this happened in a busy car park on a Sunday afternoon and there may be people with footage on mobile phones and I would urge those people to come forward."