A man and a woman have been arrested on suspicion of murder after a two-year-old boy died in hospital, police have said.
Officers were called to a house on Bosworth Road in Adwick, Doncaster, at about 11am on Wednesday following reports from the ambulance service that a toddler was in cardiac arrest.
He was taken to hospital where he died on Thursday afternoon, South Yorkshire Police said.
A post-mortem examination was due to be held on Thursday evening.
The force said a man aged 36 and a woman aged 32, both from Adwick, have been arrested on suspicion of murder.
Detective Chief Inspector James Axe, the senior investigating officer, said: "The loss of any life is tragic, and the death of this little boy is undeniably devastating.
"Our thoughts are with those who are affected and our specialist officers will continue to support them.
"We have a dedicated team of officers assigned to this investigation.
"Two people are in custody and we are working hard to establish the exact circumstances leading up to this child's death."
Temporary Detective Chief Inspector Anna Sedgwick, from Doncaster, said: "I know a case such as this will have an impact both within the local community and more widely.
"We will have officers in the area today and encourage anyone who has any concerns, or would like to speak to someone, to approach a member of our team."
The senior officer urged people not to speculate online about what happened to the little boy.
She said: "I would also please ask that people are mindful of what they post on social media, particularly in relation to any speculative comments, and keep in mind the impact this could have on the boy's loved ones during this incredibly difficult and traumatic time."
Anyone with information is asked to call 101