Narumi Kurosaki disappeared in the city of Besancon, where she had been studying French, on the night of December 4, 2016.
Local prosecutor Etienne Manteaux said that determining the exact cause of death was difficult because investigators had not found her body despite extensive searches involving dogs and divers.
But he confirmed that no blood had been found in her room in a university residence in Besancon where terrified cries and the sound of a physical struggle were heard by fellow students at around 3:30 am.
"Was it by suffocation? Probably. That doesn't leave blood," Manteaux told reporters in Besancon.
He said Kurosaki's ex-boyfriend Nicolas Zepeda, who posted videos online threatening her before her disappearance, was "more than ever considered the main suspect in this killing".
Zepeda left France and returned to Chile shortly after Kurosaki was last seen dining with him in a restaurant a short drive from Besancon. He has denied responsibility for her death.
Chilean authorities have so far declined to arrest him, despite an international arrest warrant issued by France, and Manteaux admitted that there had been a "certain reticence" to take action.
French authorities will send a request to their Chilean counterparts "in the coming weeks" to ask if a French magistrate can visit the country to interview the suspect.
France had not sent an extradition request, Manteaux said, adding that it was difficult to get countries to extradite their own nationals for trials overseas.
He suggested the most likely conclusion was either a trial in Chile, based on evidence collected by French authorities, or a trial in France which would judge Zepeda in absentia.
Kurosaki's parents, who reportedly visited France last week, continue to hold out hope that their daughter might be seen again.
"For as long as the body has not been found, they continue to cling to the hope that she might be alive," he said, before adding that investigators believed she was dead given the evidence they had collected.
In their search for her body, police have focused their efforts on the vast Chaux forest area on the outskirts of Besancon.
Manteaux gave a hint of how the body might have been disposed of, saying that Zepeda had bought five litres of flammable liquid and matches at a supermarket days before the student disappeared.