The mother of a 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 told Reuters from Goa.

Keeling's mother Fiona MacKeown has repeatedly complained about the police 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 body organs were not preserved properly and the police did not ask for tests soon enough.

"It will be extremely difficult in court to get justice for Scarlett Keeling now," Varma said.

A bartender, Samson D'Souza, has been arrested and charged with raping and drugging Keeling, before leaving her to die in shallow sea water.

MacKeown's lawyer said the shack where Keeling was drugged before being raped has been pulled down.

"Whatever evidence that was left in that shack is gone and there is only sand in its place now," Varma said.

Goa police denied any role in pulling down the shack.

"Maybe the owner dismantled the shack as it is his property," Jorge said.

Keeling's body is still in a mortuary in Britain and her mother wants more tests to be done before she is buried, Varma said.

Keeling's case has thrown an uncomfortable spotlight on the safety of tourists in India, and prompted tourism officials to ask police to provide better security.