Biotrial, a French drug evaluation company based in Rennes and with offices in London and the United States, was running the trial on behalf of the Portuguese pharmaceutical company, Bial.
Three separate investigations are under way to try and find out what went wrong.
Judicial police began their investigation on Friday at the premises of Biotrial in Rennes.
They were joined on Saturday by officials from IGAS, the national health audit and evaluation office, and from the national agency for medicines and health products Safety (ANSM).
The investigators are trying to determine whether the accident was caused by an error in the testing procedure or by the drug molecule itself.
All the other people involved in the drugs test have been contacted and none have so far reported anything unusual, the hospital in Rennes said.
Dr Gilles Edan, chief neuroscientist at the hospital, said in addition to the brain-dead man who has now died, three other men could have “irreversible” brain damage.
A fifth man is suffering from neurological problems and a sixth man is being kept in the hospital but is in less critical condition, he said on Saturday.
Mr Edan said there is no known way to reverse the effects of the experimental drug, which was given orally to healthy volunteers as part of a Phase 1 trial by Biotrial.