A mother's fear that her 12-year-old daughter was being stalked by a paedophile on Facebook sparked an in-depth, almost month-long investigation involving the Australian Federal Police.

But police have been left dumbfounded by the discovery that the person who harassed and sent pornographic images to the girl via the site was another 12-year-old girl.

Some of the messages left by the stalker.

Earlier this month this website reported that a Sydney mother of three, her daughter and daughter’s friends had been subjected to a two-week ordeal at the hands of a Facebook stalker, but had been unable to get the social networking company to act.

The mother reported the matter to police.

The case was dropped this week by police after finding the stalker was another girl.

The author of this post is on Twitter: @bengrubb.