The only survivor of the Kingsmills massacre has said he believes a social media post from a Sinn Féin MP was "depraved" and "designed to hurt".
Barry McElduff posted a video on Twitter on Friday - the anniversary of the killings - in which he had a loaf of Kingsmill bread on his head.
Ten Protestants were murdered by the IRA in the massacre on 5 January 1976.
He has apologised. Party colleague Máirtín Ó Muilleoir has also apologised for retweeting the original video.
Mr Ó Muilleoir said he believed the tweet was "wholly apolitical and I retweeted it on that basis".
"Once it was pointed out that the tweet had offended and hurt people, and that Barry had rightly deleted his tweet, I retweeted his apology and, of course, apologise unreservedly for the pain and hurt caused by my retweet."
Sinn Féin has said that party leadership will meet with Mr McElduff on Monday.
The DUP has said that "major questions still stand for what action Sinn Féin leadership will take against the elected representatives who posted the video".
Ulster Unionist Danny Kennedy said he believed the video was "obscene" and "deliberate".
Mr McElduff said he had not realised there was a possible link between the brand name and the anniversary. He said it was "never my intention to offend anyone who suffered grievously".
However, Alan Black, who survived the atrocity despite being shot 18 times, said he did not believe this.
"It was like a punch to the stomach, it was so callous. To mock the dead and dance on their graves is depraved," Mr Black said.
"It was designed in my mind, it was designed to cause maximum hurt and it's done its job in spades. Bessbrook [where the victims were from] is really hurting."
He said he did not accept Mr McElduff's apology.
Apology 'not heartfelt'
"No, absolutely not, again because I don't think he realised this was going to be far-reaching," Mr Black said.
"He just wanted to hurt the Kingsmills families, but it's hurt a lot of people across the north of Ireland. He had to back-track and give a half apology."
Mr McElduff has offered to meet families of the victims, but Mr Black said he was not interested in this.
"Absolutely not. I steer clear of bigots no matter where they come from," he said.
Colin Worton, whose brother Kenneth was one of those murdered, said Mr McElduff's apology was not "heartfelt".
"He's not sorry for the video that he's posted, he's sorrier for what's happened in the aftermath," he said.
"If there's any decency left in McElduff's DNA he should resign."
Mr Worton said he first became aware of the video when his son-in-law sent him a picture of it.
"My first thoughts, I was totally sickened, in disbelief and then I was angry."
He said he did not accept that it had been a coincidence.
"Those coincidences are extremely rare, especially nearly enough on the same day as the anniversary," he said.
"If he had been walking through the Kingsmill factory and put a Kingsmill loaf on his head, yes I would accept that.
"Obviously he has walked through an ordinary store with multiple loaves on the shelf, how did he manage to pick the Kingsmill brand?"
Mr Worton said he did not see any point in meeting with Mr McElduff as Sinn Féin still cannot admit that the IRA carried out the Kingsmills murders.
Kenneth Worton, who was 24 and a father of two, was one of 10 workmen shot dead on 5 January 1976 after gunmen stopped their van and asked which among them was a Catholic, and instructed that man to leave the scene.
At the time it was claimed by a group calling itself the South Armagh Republican Action Force.
The Historical Enquiries Team of the PSNI later stated that the attack was carried out by Provisional IRA members.
'I apologise unreservedly'
In the original Twitter post, Mr McElduff says he is in a service station, then, mentioning the name of the shop, asks where they keep their bread while walking with the loaf balancing on his head.
In his apology, Mr McElduff said: "Had not realised or imagined for a second any possible link between product brand name and Kingsmills anniversary.
"Further, I apologise for any hurt or offence caused. Never my intention to offend anyone who has suffered grievously."
In a later statement, he added: "I apologise unreservedly for the hurt and pain this post has caused."
The police have said they have received "a number of reports of a post on a social media platform, made by a local public representative. Enquiries are ongoing."