A mother who murdered her seven-month-old daughter had admitted seriously injuring another child years earlier.
Social services allowed Jennifer Crichton, 35, to keep baby Amelia despite the previous attack, which was not prosecuted.
She was convicted of Amelia's murder, and cruelty and causing grievous bodily harm to the other child, at Preston Crown Court on Tuesday.
Lancashire's Safeguarding Children Board has set up a serious case review.
Police arrested Crichton over the earlier attack but never charged her because the Crown Prosecution Service (CPS) said there was not enough evidence.
She went on to admit harming the child to a social worker, a nurse and a doctor but it was only after she murdered Amelia that she was prosecuted for it.
The court heard Amelia was living with Crichton under a Lancashire County Council care plan which had been outsourced to a local care provider.
On 19 April, a social worker visited them at home in Leyland, Lancashire.
The social worker eventually left but an hour and 20 minutes later Crichton called for an ambulance.
Amelia had suffered "catastrophic" injuries including a fractured skull and died two days later.
Crichton was found guilty of Amelia's murder plus three counts of grievous bodily harm with intent and child cruelty to the earlier attack victim, who cannot be named for legal reasons.
She will sentenced after undergoing psychiatric assessment.
Det Insp Simon Cheyte, of Lancashire Constabulary, said: "The assault on the other child was subject to a previous investigation.
"Jennifer Crichton was arrested but it was decided by the Crown Prosecution Service there was not enough evidence to prosecute.
"Following the murder of Amelia, it was reviewed and further evidence was discovered.
"This was considered along with the supporting evidence of Amelia's sad death and it was decided that charges could be brought."