Garda Commissioner Martin Callinan said the entire force had been deeply shocked and saddened by the murder of Detective Garda Adrian Donohoe - the first of a serving officer in 17 years.
Commissioner Callinan said: "He was a very popular member of An Garda Siochana, a very efficient member, someone who was revered both by his peers and members of the community where he served."
A wide police cordon extending for several miles was still in place around the crime scene outside Lordship Credit Union at Bellurgan, Jenkinstown which is about seven miles outside Dundalk.
Specialist officers from the Garda Forensic and Technical Bureau were combing the area for clues and could be seen scouring the bushes and hedgerows lining the rural roads. A blue tent was erected in the Credit Union carpark where the officer was tragically cut down.
The slain officer's body was taken away last night to Our Lady of Lourdes Hospital in Dundalk where a post mortem examination was expected to be carried out.
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The pair, believed to be the boy's mother and stepfather, were held after the five-year-old's lifeless body was discovered in a lagoon.