Police on Thursday completed an unprecedented collection of DNA samples from hundreds of male students and staff at a French school where a girl was raped last year.
The one student who had refused to take part "for philosophical reasons," relented on Thursday after being warned by the prosecutor leading the investigation that he would be putting himself in a difficult position if he maintained his stance.
"As he was the only one to refuse, we made him understand that he could be seen by his fellow students as having something to hide," said La Rochelle prosecutor Isabelle Pagenelle.
Samples were taken from a total of 527 students and staff private Catholic high school Fénélon-Notre-Dame in La Rochelle who were on site when the girl was raped in a school toilet where the lights had been turned off.
As she was unable to see her attacker, investigators have had little to go on other than a male DNA trace recovered from her clothing.
Nine people who have left the school since the incident on September 30, 2013 still have to be tracked down and tested, Pagenelle said.
The samples will now be sent to a laboratory in Lyon for testing and, unless one of them proves to be a match, they will all be destroyed after analyses are completed.
School director Isabelle Deveaux congratulated the students on their cooperation with police. "We are lucky to have very mature pupils," she said.
"Many parents have expressed their support for what we are doing."
Parents had to give their consent for the DNA samples to be taken from their children via saliva swabs.
The operation has attracted huge media interest in France and some lawyers have suggested the prosecutor's approach could set a dangerous precedent in terms of civil liberties.
A number of commentators have also questioned whether the investigation might have made faster progress if the school had not hushed up the rape.
Students only became aware it had happened when the DNA tests were announced last week.