Two boys aged 13 and 14 have died after a "serious incident" at a property in Sheffield, police say.
Four other children aged 11, 10, three and seven months are being treated in hospital.
A man, 37, and a woman, 34, have been arrested on suspicion of murder.
Cause of death has not been established and a post-mortem is due to be carried out.
South Yorkshire Police said "family have been informed and are receiving support from specially trained officers".
There is "no wider risk to the community", the force added.
Officers were called to Gregg House Road in the Shiregreen area of the city at about 7.30am.
The property is believed to be about 100 metres from Hartley Brook Primary Academy, where the local air ambulance landed on the school playground.
The school is not connected to the incident, officers said.
"Detectives and specialist officers remain in the Gregg House Road area carrying out enquiries and we are working hard to ascertain the exact circumstances surrounding this incident," said Detective Chief Inspector Simon Palmer.
"We will remain in the area this afternoon and into the evening carrying out enquiries and providing reassurance.
"I appreciate the events of this morning have caused concern locally but I'd like to reassure you that there is no wider risk to the community.
"To allow the investigation to progress, I would please ask that people are mindful of what they are posting on social media and the potential distress this may cause."