A three-year-old girl did not die in a playground accident but from an "act of significant violence", police have said.
Lia Green was feeling unwell on August 30 and was taken by paramedics from her home in Norris Street, Preston, Lancashire, to hospital but was pronounced dead on arrival.
A post-mortem examination carried out by a Home Office pathologist showed she died as a result of significant violence being inflicted upon her which caused serious injuries to her internal organs.
Police want to know where Lia was and who she was with from any time on Tuesday August 28 to the time she died at 8.20am on Thursday August 30.
Lia's parents, Richard Green, 22, and Natalie Critchley, 20, were arrested on suspicion of murder and are on bail until October.
Detective Superintendent Dermott Horrigan, of Lancashire Constabulary, said: "This is a murder inquiry as Lia was killed by an act of significant violence.
"There has been talk of a playground accident causing her death but this is simply not the case. The post-mortem examination has determined that this little girl did not die as a result of a fall from a swing.
"My team of officers are working around the clock to try to establish exactly what happened to Lia to cause her death and who is responsible for causing the injuries. I would appeal for anyone who has any information or who has seen Lia in the days leading up to her death to contact us.
"This remains 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 are working with the family.
"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."