An 18-month-old baby died after falling from a sixth-floor window, prompting police to arrest a woman on suspicion of murder.
West Yorkshire Police were called to the property in Bradford, following reports a child had suffered fatal injuries.
Investigators said it quickly became apparent the little boy had died.
Detective Superintendent Nick Wallen, who is leading the investigation, said those who witnessed the incident had been left "deeply traumatised".
"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," he said.
"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 that occurred on Saturday afternoon but have appealed for witnesses or anyone with information to come forward.
