Male students and staff at a high school in western France - 527 people in total - are being asked to give DNA samples as police search for an attacker who raped a teenage girl.
The La Rochelle prosecutor's office said tests are expected to last from today until Wednesday at Fenelon-Notre Dame high school, where the 16-year-old was sexually assaulted in a dark toilet on September 30 last year.
Police recovered genetic material from the girl's clothing, but there were no matches in the national DNA database.
"This happened during the school day in a confined space," Chantal Devaux, the private Roman Catholic school's director, said in interviews with French media.
"The decision to take such a large sample was made because it was the only way to advance the investigation."
Summonses went out last week to 475 students, 31 teachers and 21 others - either staff or males who were on campus at the time. The cost to analyse the samples is estimated to total 5,000 euros (£4,150).
Ms Devaux acknowledged that all the results could come back negative, sending investigators back to the drawing board.
France has an extensive DNA database, with a total of two million profiles kept on file as of 2012, and refusing a test can be used against suspects in court.