A mother who helped arranged a cruel internet hoax that apparently drove a 13-year-old girl to suicide has escaped conviction on charges that could have put her in prison for 20 years.

Lori Drew, 49, from Missouri, was instead convicted of only three misdemeanour offences of accessing computer without authorisation. Each is punishable by up to a year in prison and a $100,000 fine.

Prosecutors, who described the trial as the first “cyber-bullying” case, said that Ms Drew and two others, her assistant, Ashley Grills, 18, and her daughter, Sarah, 13, created a profile of a fictitious 16-year-old boy on MySpace, the social networking website, and sent flirtatious messages from him to a teenage neighbour, Megan Meier.

They named the boy Josh Evans and posted a photograph of him on his fake profile page, in which he appeared bare-chested and with tousled brown hair.

Ms Drew then had their fictitious boy “dump” the girl by saying: “The world would be a better place without you.”

Megan promptly hanged herself with a belt in her bedroom closet.

Thomas O'Brien, the chief federal prosecutor, in his closing argument, said: “Lori Drew decided to humiliate a child. The only way she could harm this pretty little girl was with a computer.

"She chose to use a computer to hurt a little girl, and for four weeks she enjoyed it.”

During the trial, it was claimed that Ms Drew wanted to hurt Megan for saying unkind things about her own teenage daughter. It was also claimed that Ms Drew knew that Megan suffered from depression and was emotionally fragile.

Nevertheless, a federal jury in Los Angeles rejected three felony charges against Ms Drew of accessing computers without authorisation to inflict emotional harm, and the judge, George Wu, declared a mistrial on another charge of conspiracy.

The case hinged on an unprecedented interpretation of computer-fraud law.

Ms Drew was not directly charged with causing Megan's death. Instead, prosecutors indicted her under the federal Computer Fraud and Abuse Act, which in the past has been used in hacking and trademark theft cases. Among other things, Ms Drew was charged with conspiring to violate the fine print in MySpace's terms-of-service agreement, which prohibits the use of fake names and harassment of other MySpace members.

Missouri authorities said there was no state law under which Drew could be charged. But federal prosecutors in California claimed jurisdiction because MySpace is based in Beverly Hills. After the suicide, Missouri passed a state law against cyber-harassment.

Similar federal legislation has been proposed in Washington.

Ms Drew's lawyer, Dean Steward, said: “I don't have any satisfaction in the jury's decision. I don't think these charges should have ever been brought.”

Tina Meier, the mother of the dead girl, said: "For me it's never been about vengeance. This is about justice.”

MySpace, which is a division of News Corporation, owner of The Times, said in a statement that it “respects the jury's decision and will continue to work with industry experts to raise awareness of cyber-bullying and the harm it can potentially cause.”