Police were questioning a man today following the death of a 15-month-old girl injured in an alleged assault.
The girl was under intensive care yesterday at the Royal Victoria Hospital in Belfast after being hurt in Enniskillen, Co Fermanagh.
Police arrested a 30-year-old man as part of the investigation.
Detectives from the PSNI's serious crime unit are involved and searched a house at Glebe Park in the Chanterhill area of Enniskillen.
The child was first admitted to the Erne Hospital in the town last night and then transferred to Belfast.
Western Health Trust staff are assisting police.
Glebe Park is a residential area of the lakeside town on the road to Tempo. Forensics staff have been combing the area for clues.
A police spokeswoman said: "Police have launched a murder investigation following the death of a 15-month-old female child in Enniskillen.
"One male has been arrested."
Police are appealing for witnesses who may have noticed anything in the area last night between 7.30pm and 8.30pm to contact them.
Local nationalist SDLP Assembly member Tommy Gallagher said: "My thoughts go out to the family. There is a great sense of shock and anger about this. The whole thing is playing out very badly."