Four children “rescued” from a house in Sheffield in the same incident in which two boys died have been released from hospital, police said.
South Yorkshire Police said they received “reports of concerns for safety” of people at an address in Shiregreen at 07:30 BST on Friday.
Six children were taken to hospital but the boys, aged 13 and 14, died.
A 37-year-old man and a 34-year-old woman have been arrested on suspicion of murder.
Four more children, aged eight months, three, 11 and 12, received treatment at hospital. Police said they were now fit enough to be discharged.
The children cannot be identified for legal reasons.
Floral tributes and balloons have been left at the scene.
South Yorkshire Police said extra officers were patrolling the area to reassure people, although police stressed it was an “isolated incident” with no wider threat to the community.
Supt Paul McCurry said officers were not looking for anybody else in relation to the deaths, and urged people to be “mindful” of speculating online.
Post-mortem tests on the boys who died had been due to take place on Friday.
Neighbours reported seeing more than a dozen police cars in the street on Friday morning, and Yorkshire Air Ambulance confirmed it had landed in the playground of a nearby school.
Gill Furniss, Labour MP for Sheffield Brightside and Hillsborough, has said she is “deeply saddened by the tragic incident”.