Mary Boyle disappeared aged six in Donegal in 1977.
Her twin sister Ann Doherty along with Margo O’Donnell made formal statements to gardaí almost two months ago, saying they were told the identity of the killer on several occasions by someone who knew Mary.
The women also say they have expressed their concerns to gardaí many times through the years but no action was taken.
Last October, Ann told the police who she believes murdered Mary and that there was political interference during the initial investigation in the days following her sister's disappearance.
The killer of this child has been protected by Gardai for 38 years. Now they target those who seek justice for her pic.twitter.com/aABwePDqle
— Gemma O'Doherty (@gemmaod1) December 18, 2015
When was Mary last seen?
Journalist Gemma O’Doherty spoke to Newstalk Lunchtime today about the disappearance and what she and the family know about the case.
"Mary was visiting her grandparents, extremely remote farm outside Ballyshannon, just four miles from the border with Fermanagh.
"This part of the country was one of the most heavily policed at the time. There were at least three permanent Garda checkpoints operating 24/7 because this was the height of the Troubles in Northen Ireland and on the northern side, the RUC and the British Army had a number of units there, which were operating permanently.
"Really people need to know the landscape that we're talking about, the nearest road was a long distance away.
"There were three cars accounted for on the day that were in that immediate area and they have been ruled out.
"It's almost 40 years that she's been trying to find her little sister, her identical twin. She suffered hugely through the years. She has taken on very serious forces in this state and stood up against them and she has suffered hugely.
Mary Boyle's twin Ann has completed her statement to Gardai. She is appealing to them to arrest the killer pic.twitter.com/9Fn8dNsied
— Gemma O'Doherty (@gemmaod1) October 1, 2015
What happened to Mary?
Gemma continued: "We should be thinking about Mary Boyle who is our youngest missing citizen.
"I believe, so does Ann and a number of officers, that she was brutally raped before her murder.
"We also believe that her rapist and murderer is still at large in this country and is a danger to other children and that is what is key about this case.
"She was a lovely, lovely girl"
Speaking about her daughter to the Pat Kenny show last year, her mother Ann Boyle commented: "She was a child that didn't like playing for too long. She was a lovely, lovely girl."
However, following the arrest her feelings are mixed: "It's heading towards 38 years since Mary went missing. That day is etched on my brain."
Following the disappearance, Ann said it was difficult for her to return to normal: "The toughest thing was coming back without her. People went back to their own lives but I can't go back to that life."
Last year, a 64-year-oldman who was being questioned about the disappearance of Mary Boyle has been released from Garda custody without charge.
Media reports today put a link between child killer Robert Black, who spent time in Northern Ireland in 1977 and died in prison yesterday, and Mary’s disappearance.
"The possibility of him coming through the border when there was such a heavy police presence, it's actually laughable if it wasn't so serious.
"It's just inconceivable that Robert Black had anything to do with it.
"The evidence points in a very clear direction and it certainly doesn't, and never did, point in the direction of Robert Black and it is very disturbing."
You can listen back to her interview here: