The girl's mother, Fiona McKeown, has been fighting to uncover what happened to her daughter since her bruised, partially clothed body was found near the shore on a Goa beach more than eight years ago.
McKeown told CNN she was "totally devastated" by the verdict. "I just feel really let down by the system," she said to affiliate CNN News 18. "I just don't have any faith in it anymore."
Officers initially said the 15-year-old drowned on Anjuna beach after taking drugs, but her mother rejected that account. She launched a campaign for a second autopsy, and investigators eventually confirmed Scarlett was sexually assaulted before her death.
Local media and a state minister said at the time that police covered up the killing
to protect Goa's tourism industry.
Goa Chief Minister Laxmikant Parsekar expressed disappointment at Friday's verdict. "It has broken my heart as well," he said, saying he hoped there would be an appeal to a higher court.
"How can I appeal?" I can't appeal," Scarlett's mother said to CNN. "I'm just a tourist here. The CBI has to appeal."
McKeown's attorney, Vikram Verma, confirmed that only India's Central Bureau of Investigation
, or CBI, can appeal the decision.
"Even if the CBI takes it to the High Court, unless it goes through fast track, we'll probably be waiting for another eight years for a ruling," he added.
'It's been a long eight years'