The police force monitoring serial sex attacker Peter Chapman before he fled to kidnap, rape and murder Ashleigh Hall has referred itself to the Independent Police Complaints Commission.
Merseyside Police officers were meant to be keeping tabs on Chapman.
But after realising he had vanished from his Kirkby home early last year, they left it nine months before issuing a national wanted alert.
On Tuesday, a force spokesman said: "Merseyside Police can confirm that an internal review was carried out following the arrest of Peter Chapman in October last year.
"Following the review, a number of procedural improvements were identified and subsequently implemented.
"However, in view of the public interest and concerns raised following the conviction of Peter Chapman and to ensure complete transparency in terms of this particular matter, the force has referred it to the Independent Police Complaints Commission."
Earlier, Home Secretary Alan Johnson called for police to "respond" to questions about the case as he expressed his "absolute" sympathy for the family of Ashleigh Hall.
He said lessons "needed to be learned" following the "terrible tragedy"
Ashleigh's mother, Andrea Hall also questioned why Chapman was free to attack her daughter and warned of the dangers of social networking sites.
She said: "The message is for people just to be careful. Please just be careful. Make sure you please do tell somebody if you are going to meet a person. That is the message, don't go on your own."
