It read: "I worked with the ambulance man your father. I am RC (Roman Catholic). He was an IRA man. We all new (sic) it."
Father-of-two Denis Mullan (36), an ambulance supervisor, was a member of the SDLP and as such was totally opposed to violence. But he was one of many SDLP members to be murdered by loyalist paramilitaries for their beliefs.
Sources close to Denise said she was very upset when she found the note inside the newspaper and this week she was still too distraught to talk about it.
"After Beattie had been jailed, she thought this nightmare would be over, that the harassment was finally over. But somehow he's got to her again - or at least that's what the authorities are saying to her.
"They say any letters that Beattie writes would have been read in detail in Maghaberry and anything offensive would be removed by the censors. So the only way to do it would be to hide a letter inside a newspaper."
The newspaper pages, containing an article about the murder of Denis Mullan which had been printed in the Tyrone Courier, were in an envelope and when she opened the pages, the offensive hand-written note fell out.
The letter had been posted in late March to a home Denise used to live in and it was only forwarded to her present address this week.
Denise immediately brought it to the police who launched an investigation.
Sources close to Denise, who is an Aontú councillor in Mid Ulster District Council and the party's candidate in next month's Assembly election in Fermanagh and South Tyrone, said: "She really wants this torture to end.
"She had to go through a lot to get Beattie convicted because he put her through hell. She thought at least now he's in jail, she would be left in peace. But it seems it's not the case."
Last November, Beattie (65), from Annaghmore, near Portadown, was banged up for 15 months for sending the threatening letter to Denise. The letter urged her to drop a compensation claim she was in the process of taking against the killer's estate.
When interviewed by police, Beattie admitted his crime, but denied his action was motivated by malicious intent.
But when the case came to court, Beattie was found guilty of a hate crime.
Garfield Beattie (64) was questioned about this sinister letter after Mid Ulster Aontú councillor Denise Mullen made a complaint to police
And District Judge Michael Ranaghan also imposed a restraining order preventing him from any further contact with the victim's family.
Denise was just four when the then 19-year-old Beattie gunned down Denis outside the family home near the Moy.
A toddler, Denise was wearing her blood-splattered nightdress when she was found by SDLP politician Seamus Mallon. She was standing over the body of her deceased father.
Denis - a 36-year-old married father of two who originally came from Bray, Co Wicklow - was caught in a hail of 17 bullets as he went to investigate a noise outside their cottage at College Lands, Co Tyrone, on September 1 1975.
It is believed Mr Mullan - who was Seamus Mallon's election agent - was singled out by the UVF because he objected to a loyalist stealing the vote of a deceased relative at an election count.
In February 2018 Beattie was interviewed by the Sunday World at his home, where he showed us his array of deadly hunting bows he kept in his shed.
The former British soldier claimed that by the time he was arrested in connection with UVF crime, he was intending to quit his involvement with Protestant paramilitaries.
And when top detective Maurice Neilly told him his Catholic-born mum had turned up at the police station with clean clothes for him, he began to cry.
Minutes later, Beattie told the officer everything, including the location of the Sterling sub-machine-gun he used to kill Denise Mullan's father.
Four months before, the weapon - which had been stolen from a UDR base at Glenanne in south Armagh - was used to gun down the three Reavey brothers in their family home at Whitecross.
In total, the machine-gun claimed the lives of 11 Catholics. And in the Crown Court in Belfast on September 12 1987, Beattie was handed three life sentences for killing three of them.
Judge Turlough O'Donnell told Beattie he wouldn't stipulate a fixed tariff due to his low IQ. He had been used by others, said the judge.
In a remarkably frank interview with the Sunday World, Beattie admitted that for the first four years of his sentence, he "spoke" to his victims every night when he went to bed.
Beattie said he had been recruited into the UVF by veteran Annaghmore UVF loyalist William Corrigan. An HET report claimed Corrigan - later shot dead by the IRA - was with Beattie when he murdered Denis Mullan.
But Beattie also told the Sunday World that after he became a born-again Christian in January 1981, he came to terms with his past crimes.
"I spoke to God and apologised for what I'd done. After that I found peace. All violence is wrong, " he said.
Beattie was released after serving 16 years behind bars along with some of the most notorious loyalist killers of the Troubles, including the Shankill Butchers.
"If I'm being really honest, those were the best years of my life," he told us about being caged in the notorious Maze Prison.
After his release, Beattie started painting, selling his pictures at markets on both sides of the border.
A spokesman for the PSNI confirmed it was currently investigating a hate crime in mid-Ulster.
"Police received a report of suspicious circumstances shortly before 11.30am on February 11 in the Moy area. Inquiries are continuing," he said.
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