The shotgun found at the scene of the apparent murder-suicide in Co Wexford had been in the possession of suspected killer Diarmuid Flood for several years.
The businessman, who is feared to have murdered his wife and two children before taking his own life, was given the gun by a close relative some time ago, gardai say.
However, while Mr Flood was in possession of the gun, the relative continued to license it, a detail which led to detectives initially fearing there may have been another person involved in the shocking tragedy.
Officers located the relative over the weekend and have spoken to him informally. The man is due to be formally interviewed by officers from the Republic's National Bureau of Criminal Investigation.
The revelation comes as gardai continue to search for a motive for the tragedy.
Officers are to closely question family and friends in the belief that someone who has not yet come forward must know what triggered the tragic events.
Detectives are also preparing for a trawl of the family's medical and financial records to see if they shed any light.
The investigation was hampered last night when the postmortem examinations on the bodies of Mr Flood (41), his wife Lorraine (38) and their children Mark (6) and Julie (5) had to be halted after asbestos was discovered in their home, in Clonroche, near Enniscorthy, which was gutted by the fire on Saturday morning.
Forensic and technical examinations were also halted and will only resume this morning when officers will wear additional protective equipment.
A decision on when the postmortem will resume will also be made today.
Senior officers said it was not yet clear why Mr Flood apparently shot his wife dead and set their house on fire, claiming the lives of their two children, before turning the gun on himself.
"Whatever happened, this is a family where they was no suggestion of a dysfunction. There was no evidence of excessive drinking, no evidence of debt issues, no evidence of infidelity and no mental history," a senior garda said.
"Obviously there is a story behind what caused this, but whatever it is, it hasn't been uncovered yet. We believe there must be other people who know what pre-empted this.
"And we believe one or more people know more than they are telling us."
Up to 50 gardai are now involved in the inquiry. Last night the HSE said it could not comment on rumours that Diarmuid had visited a doctor earlier last week.
Lorraine's body was found on a bed in an upstairs bedroom with a gunshot wound to the torso, while her husband was discovered with a gunshot wound to the head in a downstairs room, where the fire was started using petrol.
The shotgun was recovered close to the body.
Little Julie's remains were discovered in her bedroom, while Mark was found on the landing. Both died as a result of the fire.
The tragedy happened just a few miles from the village of Monageer, where four members of the Dunne family died in a murder-suicide a year ago.
Mr Flood ran a water pump company from a premises behind his home.
His wife was also a director of the company, Sean Flood Water Pumps Ltd, from which the couple took a salaries last year.
Lorraine, nee Kehoe, was a first cousin of Republic of Ireland and Reading soccer star Kevin Doyle. She was a contestant in the Rose of Tralee in 1991, when her future husband acted as her escort. She also gave aerobics classes twice a week at the local community centre just yards from their home.
Her uncle, Denis Kennedy, told the Irish Independent that neither the Flood nor Kehoe families knew what triggered the tragedy.
"They were a model family and a gorgeous couple. There was no issue there that we are aware of. Maybe in the fullness of time something will be discovered," said Mr Kennedy, who has acted as a family spokesman since the tragedy.
A tearful Mr Kennedy said both sides of the family were grieving together and that no-one had pointed a finger of blame.
He said Diarmuid's parents, Sean and Kathleen, and Lorraine's parents, Jim and Kathleen, "will never get over the shock of it".
"We are a tight-knit community and all we can hope is that this community spirit and people's prayers can get us through it," he added.
Several neighbours said the Floods looked every inch the happy family.
"They looked like a real partnership. I remember Diarmuid would drop the children to school and Lorraine would pick them up in the evening," said one local woman.
Norma Doyle, principal of Clonroche National School, where Mark was in senior infants and Julie in junior infants, told the Irish Independent that psychologists were being made available for students affected by the loss of their schoolmates.
"We are all devastated and deeply saddened by the events of the past few days," she said.
"This is a very close-knit community and we are struggling to come to terms with this dreadful tragedy.
"Our school has lost two beautiful students. They were happy, bubbly children. We will miss them dearly."
Local curate Fr Richard Roche, who gave the last rites to the family, said the deaths had "numbed and saddened" the entire community.
Taoiseach Bertie Ahern offered his condolences to both families.
"It would really, I think, shake anybody's heart. We've just seemed, in the last few years, to have had a number of these tragedies and all as I can is give my deepest of sympathy to the relatives and close friends of the family," he said.
Little Mark Flood was honoured with a student award in Clonroche national school just days before his tragic death.
Both Mark (6) and his sister Julie (5) were popular students at the school, which is located less than 100 metres from the gutted two-storey house in which the children's bodies were found on Saturday morning.
The revelation came as it emerged that full counselling and psychological support services will be provided at the Wexford primary school today for friends and classmates devastated by the tragedy that claimed the children's lives alongside their parents, Diarmuid (41) and Lorraine (38).
School principal Norma Doyle admitted it has been a traumatic time for everyone in the village and she described Mark and Julie as "absolutely beautiful students".
"On behalf of the staff and board of management of Clonroche national school, I would like to take this opportunity to extend my deepest sympathy to the Flood and Kehoe families at this most difficult time," she said.
"We are all devastated and deeply saddened by the events of the past few days. This is a very close-knit community and we are struggling to come to terms with this dreadful tragedy."
She added: "Our school has lost two absolutely beautiful students — Julie was in junior infants and Mark was in senior infants.
"They were happy, bubbly children and we will miss them dearly."
The school is now deeply conscious of the potential upset among both classmates and friends of Mark and Julie. "As a school body our priority now is to look after the well-being of all our students and our school community.
"To this end, we will be implementing our critical-incident plan," she said.
"We will also have psychologists available to offer advice and support."
The principal then appealed to the media to respect the traumatic time that the school and its students are now going through.
"If I could just request that when the school opens the media will respect the sensitivity of this situation and consider the vulnerability of our students at this time," she said.
As locals in the Co Wexford village of Clonroche struggled yesterday to come to terms with the horrific death of a popular family of four, there was an air of disbelief along the Wexford village's main street.
The normal air of busy commerce had been replaced with an eerie quiet as gardai manned roadblocks around the gutted two-storey house in which four people died.
Children's clothes hung on the washing line in the garden, but the garda cordon around the Flood family home pointed to the tragedy that had occurred within.
Locals tried to studiously avoid the media, while still clearly intrigued by developments in the case. Around the TV satellite vans, curious children gathered to watch.
A 200-metre section of roadway directly outside the Flood's two-storey house — with half its roof burned off and scorch marks around its windows — remained sealed off as garda technical experts continued their examination.
Throughout the day, garda activity at the house remained intense as technical experts worked to determine the cause of the fire and the terrible sequence of events that claimed four young lives.
The tragedy was merely underlined as, every so often, gardai on duty outside were asked to accept wreaths and floral tributes from shocked locals.
The deaths of Diarmuid (41) and Lorraine (38) Flood and their two children, Mark (6) and Julie (5), hung like a dark cloud over Clonroche and nearby Enniscorthy.
After so many terrible tragedies in Wexford over recent years, Clonroche locals were in shock that the latest horror should occur on their very doorsteps.
Sympathisers, well-wishers and neighbours called by throughout the day at the homes of relatives of the family many carrying flowers and Mass cards.
Yesterday morning, the entire community prayed for both families at a special service in Cloughbawn Church, just outside Clonroche.
The congregation were told: "It is with a great sense of sadness and loss that we gather here this morning. As a Christian community here we are united in sadness and sorrow at these tragic deaths. Our prayers and our thoughts this morning are with the Flood and Kehoe families from this parish. It has left us all numb."
Clonroche's two clergymen, Fr Richard Redmond and Rev Roger Harmsworth, vowed to do everything to rally the community to support both families and acknowledged that the community already had experience from previous tragedies.
Fr Redmond was called to the smouldering house on Saturday to administer the Last Rites to the family.