The bullets were fired from weapons of the same caliber, but investigators found they had different groove marks, said Marcelo Baez, a spokesman for the prosecutor's office in the state of Salta.
Hikers discovered the bodies of French university students Houria Moumni, 24, and Cassandre Bouvier, 29, on July 29 in the San Lorenzo nature reserve overlooking the northern city of Salta, capital of the state of the same name.
Police said that Bouvier was on her knees when she was shot in the head, while Moumni was shot twice in the back.
On Saturday police found a .22 caliber rifle in the home of a neighbor of one of the suspects – and ballistics experts say this weapon fired the shot that killed Bouvier.
But police now believe that there was more than one killer and are searching for a second murder weapon, Baez said.
Moumni was shot twice from behind, according to the autopsy. One bullet went through her shoulder, while the other grazed her arm. The bullets were found on the ground nearby.
That means that those bullets "are from a different weapon," even though they are of the same caliber, Baez said.
Police believe that Moumni bled to death from her wounds.
One of the seven suspects arrested in the case, police cadet Federico Canizares, 23, entered a not guilty plea before a local judge on Thursday.
Canizares's sister, Maria Fernanda Canizares, 24, is also under arrest because she had a mobile phone and a camera belonging to one of the victims.
She said it was a gift from her boyfriend, Gustavo Lasi, 24.
Lasi, a part-time tour guide, is being held in connection with an attempt to hide the murder weapon.
The first suspect arrested in the case, construction worker Daniel Vilte, 23, was detained on August 2 on charges of trying to buy or sell the murder weapon, even though he did not have it at the time of his arrest.
He now faces charges of double homicide and rape, according to his lawyer. He has denied all charges.
Vilte said that on July 15, the day the French women entered the nature reserve, he was at home watching television. His wife, however, testified that between 6pm and 8pm Vilte was not in the house, Baez told AFP.
Investigators expect to determine the exact time of death in about a week, data considered key to determining whether the two women were held captive before being killed.
The murders have shocked residents of Salta, an otherwise quiet city of 650,000 which depends heavily on tourism.