Gardai are set to interview relatives of a nine-week-old baby to establish how she sustained serious injuries.
There are grave concerns over the tot's condition, which is critical, and the little girl continues to be treated at Temple's Street Children's Hospital in Dublin.
The baby, from Co Louth, was rushed to hospital on Tuesday by ambulance after family members raised the alarm.
The newborn sustained serious head and body injuries, sources said.
Investigators quickly became concerned regarding the extent of the injuries the baby sustained and do not believe it was a result of an accident.
A source said: "The injured baby has sustained serious injuries that are unexplained. It's very early stages in terms of the investigation but it is being taken very seriously."
Some relatives of the baby have spoken to gardai since she was admitted to hospital.
However, cops will now carry out interviews as they continue to establish the facts of the case.
The garda investigation team are also liaising with medics in the hospital as part of the probe.
A statement from garda headquarters confirmed investigators from Drogheda Garda Station are leading the investigation.
A spokesman said: "Gardai in Drogheda are investigating all the circumstances which led to an infant being admitted to CHI, Temple Street, on the 13th of September, 2022.
"The infant is currently in a critical condition. No further information is available at this time."
Tusla, the child and family agency, would not comment on the specifics of the case but it has been notified.
A spokeswoman said: "Any trauma or injury to a child is very difficult for the family and community.
"However, it’s not appropriate for Tusla to comment at this time, while all the facts are not known.
"When a child or family enters into a relationship with a public service such as Tusla they are entitled to expect that information generated in that relationship is treated in confidence and remains private.
"This is critically important in the subject matters which Tusla is involved.”
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