New Delhi: The mother of a British teenager murdered in the Indian state of Goa fears the case against her daughter's killer is falling apart after a forensic test failed to find evidence of rape, her lawyer said on Wednesday.

Fifteen-year-old Scarlett Keeling was found dead on a beach in the coastal state of Goa on February 18 this year.

Police originally said Keeling had drowned after taking drugs but changed their story after Keeling's mother complained, and a second autopsy concluded she had been raped and murdered.

Goa police now say forensic experts did not find any traces of semen in the vaginal swab they had collected for examination.

"Technically it means she was not raped and we will have to bank on other evidence like witness accounts and police statements to back our case," Bosco Jorge, a senior police officer said from Goa.

Keeling's mother, Fiona MacKeown, has repeatedly complained about the police's handling of the case. "She sounded very stunned when she heard the latest news and said the case seemed to be falling apart," her lawyer Vikram Varma said. "I think the case is falling apart with all evidence being tampered by the police," Varma said, adding that body organs were not preserved properly and the police did not ask for tests soon enough.

A bartender, Samson D'Souza, has been arrested and charged with raping and drugging Keeling, before leaving her to die in shallow water. Varma said the shack where Keeling was drugged has been pulled down.