PARIS—A clinical drug trial seriously harmed five men, leaving one of them brain dead, in what France’s top public health official described on Friday as an unprecedented accident.

French Health Minister Marisol Touraine said the men were part of a group of 90 people participating in the clinical trial of a new drug developed by the Portuguese pharmaceutical company Bial.

“We have never seen any trace of a similar accident,” Ms. Touraine said, pledging to “shed light on this tragic accident and find all those responsible.”

The men, between 28 and 49 years-old, were sent to a hospital in the northwestern city of Rennes. One was declared brain dead, while the four others suffered neurological disorders. A sixth man enrolled in the trial has been hospitalized and is under observation, though health officials weren’t more specific about his condition.

On Friday, Paris prosecutors launched an investigation into how the men, who were healthy volunteers, suffered serious harm, a spokeswoman for the prosecutor’s office said.