"We were not able to determine the genetic profile (of a perpetrator) from DNA taken from the body of Cassandre Bouvier, because there were several rapists," said the source, citing a report issued by a genetics lab run by the University of Buenos Aires.
"With such a mixture of DNA it was impossible" to determine the identities of the individual rapists who took part in the crime, the source told AFP.
Officials, however, are still awaiting results from DNA tests on strands of hair that were found in hands of one of the victims, presumably from one or more of the rapists.
Authorities said on Wednesday that DNA evidence linked a 24-year-old Argentinian tour guide, Gustavo Lasi, to a sexual assault of Bouvier's travel companion Houria Moumni.
Both women were fatally shot while on holiday last month in a nature preserve in northwestern Argentina.
Hikers discovered the bodies of Moumni, 24, and Bouvier, 29, on July 29 in the Quebrada de San Lorenzo nature reserve overlooking Salta, the capital of the state of the same name.
Bouvier was shot in the head while Moumni was shot in the back. Witnesses told investigators they heard gunshots on July 16 near where the bodies of the French students were found.
Ballistics evidence had already shown that a rifle belonging to Lasi was one of two weapons used in the murders, according to the police.
Lasi is among eight suspects detained in the killings, none of whom have yet been charged with any crime.
The others are his father Walter Lasi, 42, his girlfriend Maria Fernanda Canizares, 24, her brother Federico Canizares, 23, another tour guide Dario Ramos, 47, gardener Santos Vera, 37, construction worker Daniel Vilte, 24, and farm worker Raul Sarmiento, 45.
The double murder has shocked residents of Salta, an otherwise quiet city of 650,000 that depends heavily on tourism.