A note saying “Call 999, don’t go upstairs” was left on the window of a house in which the bodies of three young children were found on Friday night.
The two boys and a girl were found in a house in the Parson’s Court estate in Newcastle, Co Dublin.
The three children aged 4, 8 and 10 were found dead when emergency services were called shortly after 7pm.
The children’s mother was taken to Tallaght Hospital where she was being treated on Friday night.
It is understood the tragedy was uncovered after a concerned taxi driver picked up the woman who was in a distraught state nearby and brought her home.
There he discovered a note on the window, saying: “Call 999, don’t go upstairs.”
Emergency services and Gardai went to the house after being alerted.
When officers arrived at the scene, they found a woman outside the house.
The woman, who is in her 40s, was said to have looked dazed and disorientated.
Inside the house, officers found the bodies of the three children.
The cause of the children's deaths was not immediately known, but Gardai said they did not believe they died from natural causes.
A senior source said there was "no sign of trauma" on the bodies of the children.
Gardai confirmed they were not looking for any other person in relation to the deaths at this time and said there was no sign that the house had been burgled.
A post-mortem will determine the cause of death.
The area around the home at Parson's Court, a mixed complex of houses and apartments, was sealed off by Gardai pending an examination by technical experts.
Shocked neighbours huddled outside their doorways in the housing estate, stunned by the devastating news.
A neighbour who parks her car directly in front of the house where the bodies were found said she didn't know the family other than to exchange pleasantries.
She said she came home around 6.45pm and saw a woman slumped over on the road with a taxi parked beside her.
"She looked like she was after collapsing," she said. "I saw that woman there and I thought she was knocked over."
The woman said that shortly after this the woman was taken away by an ambulance.
She said the estate was normally very quiet.
Local resident Austin Cooney (47) anxiously stood outside the large Garda perimeter that cordoned off the housing estate.
Mr Cooney is a coach with local soccer club Peamount United and has three children himself under the age of 10.
He said that, as a parent, the news was simply horrific.
"It's horrible. It's devastating," he added.
Gardai asked for anybody with information to contact Clondalkin garda station at 01 6667600, where an incident room has been established, or the confidential line on 1800 666111.
Emergency responders are expected to be offered trauma counselling.