A woman has been arrested on suspicion of murder after an 18-month-old baby died following a fall from a sixth-floor window.
Officers were called to Barkerend Road, Bradford, at about 5.10pm on Saturday afternoon following a report that a child had suffered life-threatening injuries.
It quickly became apparent that the 18-month-old boy had died, West Yorkshire Police said.
Detective Superintendent Nick Wallen, who is leading the investigation, said: "This is clearly an extremely traumatic incident and specially trained officers are working to support the child's family members and those who witnessed what took place.
"It is no exaggeration to say those who witnessed this incident will have been deeply traumatised by what they saw.
"A 23-year-old woman has been arrested on suspicion of murder in connection with the baby's fall and she is currently receiving medical assessment in custody."
Police said they are not looking for anyone else in connection with the incident but have appealed for witnesses or anyone with information to come forward.
A crime scene remains in place at the scene as inquiries continue.
A police officer was standing on a landing of the Y-shaped block of flats.
Newcastle House, which has shops on the ground floor and seven stories of flats above, stands above Bradford city centre.
Entry to the building is controlled by intercom.
Anyone with any information is asked to contact Bradford CID on 101, quoting reference number 1506 of October 21 2017.