Internet search companies such as Google have come under pressure to block child pornography after a children's charity said that the sites "fuel the fantasies" of paedophiles who then sexually assault children.
As Mark Bridger was jailed for life for the abduction and murder of 5-year-old April Jones, the NSPCC said there was a "worrying link" between his looking at indecent images online and the crime he went on to commit.
Bridger's laptop contained a cache of images of children being raped and abused. Police found a horror film in his video recorder paused at a violent rape.
Last month, Stuart Hazell was jailed for the murder of Tia Sharpe, his partner's 12-year-old granddaughter. During his trial the Old Bailey heard that he had used his computer to search for terms including "violent forced rape" and "incest".
Bridger, like Hazell, had no previous convictions for sexual offences. Both went from viewing indecent images straight to the worst class of offending. With no gradual escalation in behaviour, there was nothing to suggest they were a threat to children and to alert police.
Child protection charities say web companies could introduce online warnings, threatening possible prosecution when users attempt to access explicit sites.
There have been calls for Google to introduce their "safe search" option as a default setting, which would automatically block hard-core pornography and make it far more difficult for children to access accidentally.
John Carr, the British Government's adviser on internet safety and secretary of a children's charities coalition on the subject, has said: "Google can do more and should do more."
Scott Rubin, Google's director of communications and public affairs, said: "Google has a zero-tolerance policy on child sexual abuse content. When we discover child abuse imagery or are made aware of it, we respond quickly to remove and report it to the appropriate law enforcement authorities."