Police monitoring of a sex offender who killed a teenager he met on Facebook is to be investigated by the Independent Police Complaints Commission (IPCC).
Merseyside Police waited nine months before issuing a wanted alert for Peter Chapman, 33, after its officers lost track of his whereabouts.
Chapman was jailed for life for raping and murdering Ashleigh Hall, 17, in a field in County Durham, last October.
Ashleigh's mother, Andrea, called for tighter monitoring of sex offenders.
She branded her daughter's murderer "inhuman" and said authorities should reveal where sex offenders live.
Merseyside Police referred its handling and review of the Chapman case to the IPCC after Home Secretary Alan Johnson demanded answers.
'Very great' danger
He called on police to "respond" and said lessons "needed to be learned" following Ashleigh's murder.
Ashleigh Hall's mother, Andrea: IPCC review "too little too late"
After Chapman was jailed at Teesside Crown Court, on Monday, it emerged police lost track of him when he left his home in Kirkby, Merseyside, 10 months before the murder.
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.
Sentencing him, Judge Peter Fox said Chapman was, and had been for a "considerable time", a "very great" danger to young women.
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.
Merseyside Police confirmed a 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.
Speaking about the IPCC investigation, a Merseyside Police spokesman said: "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... and to ensure complete transparency in terms of this particular matter, the force has referred it to the Independent Police Complaints Commission."
In October last year, Ashleigh was attracted by a picture of a young, bare-chested man that Chapman - calling himself Peter Cartwright - had posted on the website.
Later that month, she told her mother she was going to stay with a friend. Text messages showed she thought she was being picked up by 'Peter Cartwright's' father.
Chapman raped and killed her, before dumping her body in a gully, close to Old Stockton Road near Sedgefield. He was arrested the following day.
Whether we can get the technology to flag up when [sex offenders are] online is something we need to look at
Home Secretary Alan Johnson
During an appearance on ITV's This Morning, Ashleigh's mother Andrea Hall called for the addresses of sex offenders to be made public.
She said: "He took my daughter. He shouldn't be allowed human rights; he's not human, is he?
"He murdered my daughter. She was 17 and he knew exactly what he was doing and there was no remorse whatsoever.
"He had it planned... and she was such a lovely girl and that's probably why he targeted her."
Mrs Hall added she would never let her remaining three daughters, aged between one and six, use social networking websites.
Mr Johnson said the government was looking at ways to alert authorities when convicted sex offenders were online.
He said: "What our people in the Child Exploitation and Online Protection (Ceop) agency do is go online themselves to try and lure in these people.
"Whether we can get the technology to flag up when [sex offenders are] online is something we need to look at."
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