THE two-year-old sister of an infant gardaí believe was murdered following a house party in Lusk four weeks ago is missing and police searches are being carried out in the UK in a bid to locate the toddler.

The two-year-old sister of baby Johnny McCarthy is no longer in the care of her parents Winnie and John McCarthy, who live at the St Francis Park halting site in Navan, Co Meath. The HSE has sought access to the child but has established that she is no longer being cared for by her parents.

Gardaí have searched for the child in Ireland since the death of her infant brother but the toddler has not been located. The family previously lived in the UK – the couple's two other children were taken into state care in England in recent years and have since been adopted.

Detectives believe the toddler may have been taken to the UK and are liaising with police, who are carrying out searches at several premises in a bid to locate the child.

Detectives in north Dublin initially treated the death of 10-day-old Johnny McCarthy as accidental, then suspicious, before it was upgraded to murder last week following the post mortem and on receiving additional medical evidence.

The infant died from blunt force trauma to the head. He had also sustained bruising to his body and brain. Gardaí have ruled out the possibility that the fatal injuries were sustained during a one-off accidental fall or that the head injuries were sustained after the child was dropped as he was being held. Investigating detectives believe that the infant's injuries are consistent with being thrown. Detectives investigated a claim that the baby was dropped by another child at the house but this has not been corroborated by medical evidence.

Winnie and John McCarthy were visiting friends who were having a party in Lusk on the night of 7 January. In the early hours of the following morning, the infant sustained his injuries and it was decided he needed medical attention. He was taken by family members to Our Lady of Lourdes hospital in Drogheda, Co Louth. He was pronounced dead at 2am, not long after arriving there. He had been born at the same hospital just 10 days earlier.

His death was reported to gardaí who sealed off the house at Scholar's Walk, Lusk, where the party had been held. The property underwent a full forensic examination by members of the garda technical bureau. Gardaí believe the baby's fatal injuries were sustained just before his death and were not historical. Senior detectives say they are awaiting further forensics and ballistics results, which are expected to be pivotal to the garda investigation.

A file is being prepared for the DPP and gardaí are hopeful of securing a murder charge. A senior source said the investigation is a "slow burner" and the awaited forensics and ballistics results will be integral. Eight people in total have been arrested by gardaí in the course of their investigation, six for withholding information and two in relation to the murder.

"We know some people know what occurred on the night and would be able to assist us. We would ask them to rethink their reluctance to come forward and talk to us. Anything they would say would be treated in the strictest confidence," Chief Superintendent Gerry Phillips of the Dublin northern division said last week.

Anyone with information is asked to contact Balbriggan garda station on 01-8020510, or the garda confidential line on freefone 1800 666 111.