Gardaí are investigating an incident in Tallaght in Dublin, in which a teenager and her younger brother and sister died.
The victims have been named as Lisa Cash, 18, and twins Christy and Chelsea Cawley, both aged eight.
At around 12.30am this morning, gardaí were alerted to what they described as "a violent incident" which was taking place at a house in the Rossfield Estate in Tallaght.
The mother of the children, who was not injured at the scene, was taken to hospital and received medical treatment. She has since been released and gardaí say she is being supported by her family.
A teenage boy, aged 14, was also injured in the incident and is continuing to receive treatment in hospital. His injuries are serious but not life-threatening.
Post-mortem examinations are due to take place this afternoon and tomorrow by State Pathologist Dr Sally Anne Collis.
A man in his 20s was arrested at the scene and is currently being detained at Tallaght Garda Station under Section 4 of the Criminal Justice Act 1984.
Members of the Garda Technical Bureau are carrying out a forensic examination of the scene.
In a statement earlier, gardaí said: "Following an intervention by Garda Armed Support Unit members, involving the discharge of less-than-lethal devices, a male in his early 20s was arrested at the scene."
Gardaí said they were not looking for anyone else in relation to the incident at this time.
A garda statement: "This violent, challenging and traumatic incident was responded to by uniform unarmed garda first responders from Tallaght District, supported by uniform members and armed detectives from the wider DMR South Division and Specialist Armed Support Unit members. An Garda Síochána has put in place welfare supports for, and will continue to support, all members who attended this incident," said a garda statement.
In a tweet, Minister for Justice Helen McEntee said it was "a tragedy so awful and heartbreaking it is difficult to comprehend".
"My thoughts are with the family and the local community as they deal with this unimaginable crime," she said. "Gardaí and other services are supporting them at this difficult time."
A statement from the local primary school St Aidan's SNS said the school community was deeply saddened by what has happened.
Principal Tomás Hayes said: "Our thoughts and prayers are with the families and friends of the deceased.
"As a school, our priority will be to take every step to ensure that the needs of our students and their families are met, at this challenging time.
"We will be providing support to pupils and school staff to help them deal with this tragedy and have requested the support of the National Education Psychological Service in this respect," he added.
A spokesperson for the Minister for Education said that the National Education Psychological Service is providing support to pupils and school staff of the school to help them deal with the tragedy.
Local councillor Charlie O'Connor said it will be hard for the community in the aftermath. He said the area has been left shocked and devastated.
One man said few words can describe this incident and the impact of the lives lost. He said there was panic and disbelief on the street.
Meanwhile, friends and neighbours of the twins and their older sister have been leaving flowers and notes near the house and expressing their sadness.
One garda helped two young boys to paste posters they had made for their friends onto a wall near the house where they died.
Locals have described their hearts being "broken" and are finding it difficult to believe three young lives have been lost.
