A young girl remained critically ill in hospital last night after she suffered stab wounds and her baby brother died in their home in north Belfast.
The children's mother (29) was being questioned on suspicion of the murder of her son and attempted murder of her young daughter.
The two-month-old boy and his sister, aged two-and-a-half, were found in a house in Brompton Park in the Ardoyne area of north Belfast after 8pm on Tuesday.
Baby Liam was pronounced dead at the scene. His sister Kayla remained in intensive care last night.
Their mother has also been named locally as Raluca Tagani.
She is understood to have suffered injuries to her throat and was arrested at the house.
Police are not looking for anyone else in connection with the baby's death.
Ms Tagani, originally from Romania, moved into the area with her partner, from west Belfast, around a year ago.
However, it is believed he moved out of the house in recent days.
He was understood to have been in England but flew back to Belfast yesterday to be with his daughter.
PSNI Detective Chief Inspector Chris Millar said police found the children in the house on Tuesday evening.
"A 29-year-old woman who was also in the house has been arrested on suspicion of murder and attempted murder and remains in custody at this time," he said.
"Our thoughts are with the baby's family, who are being supported by family liaison officers.
"While our investigation is at any early stage, I can confirm that at this stage we are not looking for anyone else in relation to this tragic death.
"I would like to thank the local community for the support that they have provided and would appeal to anyone with any information that can assist our investigation to contact us."
Sinn Féin North Belfast MP John Finucane said he hoped that the toddler would make a full recovery.
"Police have arrested a 29-year-old woman, they have commenced a murder investigation also so I think that process has to take its own course," he said.
"Everybody in the community, their thoughts are with the families that are directly affected by this.
"It is a tragic situation compounded by the fact that we are dealing with somebody who has died at such a young age."
Mr Finucane added: "I think there is a senselessness to it, I think people find it very difficult to try to comprehend the facts as they are being relayed and the facts as they are emerging.
"It is incredibly difficult and will obviously leave a mark on those who are most impacted by it, and all we can do is rally round those who need support at this time.
"There were neighbours who assisted and some people who would have witnessed scenes that they shouldn't have to witness.
"Ardoyne is a very proud community, it is a resilient community also and I have no doubt whatsoever that they will rally around each other at this very difficult time, they will provide help and assistance for those who need it."
It is understood neighbours will be offered counselling.
People Before Profit councillor Fiona Ferguson said the news was "truly devastating".
"A wave of shock began in Brompton Park last night and spread out across north Belfast," she said.
"After something so shocking, community solidarity, community support, rallying around each other is going to be absolutely paramount."
