LONDON — The case was a media sensation from the start, with allegations of drug-fueled group sex and a principal suspect whose cherubic face proved to be an irresistible canvas to a world that saw in it images ranging from scheming vamp to innocent ingenue.

For four years, that contrast hovered over the fate of Amanda Knox, a 24-year-old American exchange student in Italy, trapped in a foreign legal system and behind bars for the murder of her British roommate. Was she a killer, capable of murdering Meredith Kercher in the pursuit of sexual pleasure? Or was she the helpless victim of a prosecutor’s character assassination and a botched police investigation?

On Monday, an appeals jury in the central Italian town of Perugia sided with the latter portrait. It overturned Knox’s conviction for the 2007 murder of Kercher, a British student with whom she shared an apartment. Knox’s alleged accomplice, former boyfriend Raffaele Sollecito, 27, was also exonerated by the appellate panel.

The stunning turnaround hinged on an independent review of DNA evidence that authorities said tied Knox and Sollecito to the crime. Kercher, 21, was found dead in her room, her throat slashed and her body bearing more than 40 wounds and signs of sexual assault. The DNA review found that the evidence was severely compromised by sloppy police collection methods and subpar forensic testing, a devastating conclusion that prosecutors could not successfully counter.

Two hours after the verdict was read out and beamed around the world, Knox was set free, no more to return to the cell where she has spent most of her adult life. She and Sollecito had more than 20 years left to serve under sentences handed down upon their conviction in 2009.