Hikers discovered the bodies of Houria Moumni, 24, and her traveling companion Cassandre 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 and sexually assaulted 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.
"The DNA from semen taken from the body of Houria Moumni is that of Gustavo Lasi," said Marcelo Baez, a spokesman from the prosecutor's office in Salta.
Lasi appeared in court in Salta later Tuesday as a judge sought to determine what role he played in the crime.
Ballistics evidence has already shown that a rifle belonging to Lasi was one of two guns used in the murders, according to the police.
Lasi is among eight suspects detained over 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).
According to investigators quoted in the local press, Vera and Ramos confessed to beating and raping the French tourists after attempting to steal from them but said it was Vilte who shot them. The confession, however, is not legally valid as it was not made before a judge.
Maria Fernanda Canizares was found to be in possession of a mobile phone and a camera belonging to one of the victims but she said the items had been given to her by Lasi, her boyfriend.
The two French university students had been on holiday in Argentina at the time of their deaths. The double murder has shocked residents of Salta, an otherwise quiet city of 650,000 that depends heavily on tourism.