The Paris prosecutor’s office has opened an investigation into what French Health Minister Marisol Touraine called “an accident of exceptional gravity” at the private Biotrial clinical lab in the city of Rennes. The drug trial, which was testing a new painkiller compound, involved 90 healthy volunteers who were given the experimental drug in varying doses at different times, she told reporters Friday at a news conference in Rennes. The drug was produced by the Portuguese pharmaceutical company Bial, which said Friday that 108 healthy people had already taken part in the trials and had no moderate or serious reactions to the drug, a new molecule to treat pain. The French ministry statement said those who had fallen ill had taken an oral medication in the first phase of testing, which was studying safe usage, tolerance and other measures on healthy volunteers.