The children were pronounced dead on arrival at hospital
A man held on suspicion of murdering his two children was estranged from his wife, police have said.
Theo Molemohi, aged two, and his sister Yolande Molemohi, aged four, were found unconscious by their mother at a flat in Whalley Range, Manchester.
Attempts were made to resuscitate the pair on Wednesday morning but they died in hospital later. Post-mortem examinations are being carried out.
Their 37-year-old father is being held on suspicion of murder in hospital.
Det Supt Geoff Wessell, of Greater Manchester Police (GMP), said the couple were estranged.
The children spent the weekend with their father and were being visited by their mother on Wednesday morning.
"This is a quite horrendous incident for the family and for the officers who attended," said the detective.
"What I can say is that no other person is being sought in connection with this incident at this time.
"That male remains in custody and is due to be interviewed later today."
Det Spt Geoff Wessell: "This is a quite horrendous incident for the family"
The father lived in Lansbury House, the three-storey block where the children were found, which is managed by Contour Housing.
The block is on a street in Whalley Range, a suburb two miles south of the city centre.
The housing association said it was co-operating with the police and a spokesman offered his condolences to the family of the children.
Mr Wessell said the children's mother, believed to be studying for a PhD, was "extremely distressed" and is being comforted by specialist family liaison officers.
"They [the children] were found in a bedroom with the father there next to them. He lived at the address," he added.
"She had gone back for clothes and let herself in and was confronted by this traumatic scene."
The detective said the couple, who moved to the UK from either South Africa or Zimbabwe in 2003, had some previous contact with agencies in the city.
But although they were not known to police it is not clear which agencies they were known to, he added.
Forensic science investigators remained at the scene throughout the day examining the ground floor flat and part of the gardens outside its patio door.
The results of post-mortem examinations to establish the cause of the children's deaths are expected to be released later.
Cas Page, headteacher at Our Lady's RC Primary School, said four-year-old Yolanda Molemohi was a "lovely, happy, bubbly little girl with such a wonderful smile who was loved by everyone in school".Note: Cross posted from [wp angelfury] Whos Killing Families?.