A 41-year-old woman has been arrested on suspicion of murder following the death of a child.
Officers were called to Cwm Du Close in the Gendros area of Swansea at about 20:30 BST on Thursday.
The woman is in custody in Bridgend, South Wales Police said, with the force’s major crimes investigations team looking into the death.
The force said the child and the woman lived together and officers are not looking for anyone else in connection with the death.
Gendros is part of the Cockett ward in west Swansea.
Cwm Du Close is 0.3 miles – about a seven-minute walk – away from the local primary school.
“This is a distressing incident which will be a shock to the local community,” said Chief Supt Chris Truscott, divisional commander for Swansea and Neath Port Talbot.
“Detectives are working to establish the circumstances of the child’s death and there will be an increased police presence in the area to offer reassurance to local residents.
“Speculation on social media is unhelpful and will cause distress to those affected by this incident at an already difficult time.”
‘Utter shock’
News of the child’s death has left the community reeling.
Neighbour Dominic Nutt, 46, said he was putting his son to bed on Thursday night when he saw blue lights from outside as three to four emergency vehicles raced to the street.
“There seemed to be an armed response van and police running towards the house there,” he said.
“It was very scary and there was quite a lot of activity.
“Then an ambulance came and then we saw a helicopter which landed near the library – the people living here were coming out of their houses and seemed confused.”
Mr Nutt said he has lived in the street for four years, and described neighbours being in “utter shock”.
“It’s the kind of neighbourhood that if you want to keep yourself to yourself you can, but there is a community feeling here… people here look out for each other,” he said.
“The close was like a ghost town this morning, people in shock.”
Local politicians said the “heartbreaking news” had shocked the people of Swansea and beyond.
“All our thoughts are with those affected and the community wrestling with the tragic loss of a child,” said Torsten Bell, Labour MP for Swansea West.
Rob Stewart, Swansea council’s leader, posted on social media: “This is a very distressing and tragic incident which will take time for the police to investigate and the community to process.
“Our thoughts are with everyone affected by this heartbreaking tragedy and our teams will continue to support the community at this difficult time.”
Julie James MS, who represents Swansea West in the Senedd, described it as “truly shocking and tragic”.
“My heart goes out to everyone affected,” she said.
All three reiterated the police’s request to “avoid speculation on social media about this terrible incident”.
From the scene
Meleri Williams, BBC News
Children’s toys are scattered around front gardens in Cwm Du Close on one of the last days of the school summer holidays.
But this morning it’s quiet in the residential area of Gendros in Swansea.
Behind a police cordon, a forensics tent can be seen outside a bungalow.
Police officers are busy asking neighbours for any information or CCTV.
Messages shared on social media described a large emergency service response on Thursday night.
People have been asked not to speculate about what happened as police continue to investigate.