LOS ANGELES -- A 49-year-old Missouri mother was Wednesday found guilty in a landmark "cyber-bullying" case stemming from the suicide of a teenager who killed herself after being sent taunting emails.

Lori Drew was convicted on three misdemeanor counts of illegally accessing computers without authorization but jurors at federal court in Los Angeles could not reach a verdict on a more serious charge of conspiracy.

Prosecutors say Drew faces up to three years in prison and a $300,000 fine although no sentencing date has yet been set.

Drew, who showed no emotion as the verdict was read out, was charged following the tragic death of 13-year-old Megan Meier, who committed suicide in 2006 at her home in St Louis.

Prosecutors said Drew, her daughter and a family friend who were not charged, posed as a fictitious 16-year-old boy named "Josh Evans" who befriended Meier via a bogus account on social networking site Myspace.

Meier, who Drew suspected of spreading false rumors about her daughter, hanged herself after receiving a message from "Josh" which said the world would be a better place without her, the trial heard.

The case was the first criminal prosecution in US legal history relating to allegations of cyber-bullying.

Federal attorney Thomas O'Brien said during the trial that Drew targeted Meier even though she knew the teenager was vulnerable.

"The defendant knew Megan Meier was depressed, suicidal and boy-crazy," O'Brien said.

Nevertheless Drew set out "to tease, embarrass, humiliate, make fun of and hurt her," he said.

Drew could have stopped at any time but chose not to, the prosecutor said.

"The only real adult in all this -- Lori Drew -- said, let's keep going," O'Brien said. "She was told it was illegal, but she just shrugged it off."

Drew's defense attorney Dean Steward had stressed his client was not charged with Meier's death.

"This is a computer fraud and abuse case ... not a homicide case," he said. "This was a deeply tragic case for everybody -- most of all for Megan Meier."

Steward told jurors his client was not responsible for the email from "Josh" in which Meier was told the world would be a better place without her.

Prosecutors in Missouri declined to bring a case against Drew. The case was prosecuted in Los Angeles because Fox Interactive, the owner of MySpace, is based in Beverly Hills.

Drew faced illegal access charges on the basis she lied on the fake MySpace profile created to taunt Meier, violating the site's terms of service which requires users to provide "truthful and accurate" registration information.