Euthanasia advocate took pentobarbitol, prescribed for assisted suicide in other European countries.
Chantal Sebire, a terminal cancer sufferer who had failed to change French law to allow doctors to help her commit suicide, was found dead earlier this week.
"We can say Mrs Sebire did not die of natural causes, as shown by the autopsy," said prosecutor Jean-Pierre Alacchi at a press conference in Dijon, "but from absorbing a lethal dose of barbiturate."
Her post-mortem tests showed the barbiturate Pentobarbital at three times the lethal level in her bloodstream. Pentobarbital is a drug used for animal euthanasia and is legally prescribed for assisted suicide in Switzerland and Belgium, as well as in the US state of Oregon. However, it is not available in French pharmacies.
Investigators are still not sure how Sebire obtained the drug, nor how it was administered, so there is currently debate on whether her death was suicide or assisted suicide. Her lawyer, Gilles Antonowicz, would say only that Sebire had "put an end to her own suffering. She delivered herself, but I do not wish to talk of suicide, because that was not Mrs Sebire's intention".