Dyfed-Powys Police detained a 28-year-old man for questioning on suspicion of murder, Wales Online reported.
Neighbors in Maes Yr Ysgol in Carmarthen identified the dead woman as Charmaine Macmuiris, 37. Rachel Evans said Macmuiris was a widow whose husband died several years ago from a fall, leaving her to bring up their three children on her own.
"Kielan is only 11 and was really very close to his mum. It is just completely heartbreaking," she said. "They are a really close family and none of us can take in what has happened."
Another neighbor, Garry Lowes, said he saw Macmuiris Christmas Eve when she picked up a television he had purchased for her.
"She was really looking forward to Christmas and had come around on Christmas Eve to collect the presents I had kept at my house for her," he said. "I cannot believe that hours later she was dead."
Macmuiris had recently become a grandmother.
Police have asked for anyone who was in the neighborhood between 5 p.m. Monday and 11 a.m. Tuesday to come forward.