A man and a woman are being questioned over the death of a three-year-old girl.
Lancashire Police said a murder investigation had been launched after Lia Green died in Preston on Thursday.
A 22-year-old man and a 20-year-old woman have been arrested on suspicion of murder and are being questioned by detectives.
Lia was feeling unwell on Thursday morning and was taken by paramedics from her home on Norris Street to the Royal Preston Hospital but was pronounced dead on arrival.
Officers from the Force Major Investigation Team are working to establish exactly how she died.
Det Supt Dermott Horrigan, the senior investigating officer, said: "This is an incredibly complex and sensitive inquiry and we have a team of highly trained and professional officers looking into the matter.
"Dedicated family liaison officers will be working with the family.
"The death of a child will always be greeted with shock and a deep sense of sadness.
"We have dedicated neighbourhood policing officers working in the area and talking to residents and I would urge anyone who has any concerns to speak to them."