Peter Chapman was a sex offender known to police
Merseyside Police has been criticised for not referring earlier its handling of the case of a known sex offender who killed a teenager he met on Facebook.
Peter Chapman, 33, was jailed for life for raping and murdering Ashleigh Hall, 17, in County Durham, last October.
The force had lost track of him when he left his home 10 months earlier.
The Independent Police Complaints Commission (IPCC) said it was "disappointed" it only had a referral after Monday's sentencing.
The force referred its handling and an internal review of the case to the IPCC after Home Secretary Alan Johnson demanded answers.
He called on police to "respond" and said lessons "needed to be learned" following Ashleigh's murder.
On Wednesday, IPCC commissoner Naseem Malik said its investigations would review how the force monitored Chapman, what intelligence and information was available about him and what actions were taken when it was established he was missing.
She said: "It is disappointing this matter has only come to the IPCC's attention at this late stage.
"This is a matter that should have been referred to the IPCC as soon as Merseyside Police became aware of the issues last year. That is a matter I will be taking up with the force separately.
Ashleigh thought she was meeting another teenager
"I appreciate our investigation is too late for Ashleigh's parents. They have lost a daughter they loved in horrific circumstances.
"All I can say to them is that if it is possible for any small sliver of hope to come out of this it will be for lessons to be learned that might prevent such a situation happening again."
After Chapman was jailed at Teesside Crown Court, it emerged police lost track of him when he left his home in Kirkby, Merseyside.
The court had heard that since the age of 15, Chapman had been the subject of several sexual assault investigations, including being jailed for seven years in 1996 for raping two prostitutes at knifepoint.
In the latest case, he posed as a teenage boy on the social-networking site and organised to meet Ashleigh, the jury was told.
He then suffocated her and dumped her body in a farmer's field near Sedgefield.
After he disappeared, police made attempts to trace him locally, but it was not until September, one month before Ashleigh was murdered, that the force issued a nationwide wanted alert.
The force confirmed the national alert on Chapman - who was still on parole - was only issued when he could not be contacted over a traffic offence.
Officers had last visited his house in Liverpool in January 2009.
Merseyside Police has previously said it carried out an internal review following the arrest of Chapman in October and that a number of procedural improvements were identified and implemented.
It had since referred the matter to the IPCC in view of the public interest and concerns raised following the conviction.