In Connaux, the town's baker received reports from his clients that they believed his bread was causing violent diarrhea.
The delivery driver, Jean Bousquet, sent the prefect a copy of a remark made to his employer, the miller's union in Nimes, on 9 August.
[8]: 438 On 16 August 1951, the local offices of the town's two doctors filled with patients reporting similar food poisoning symptoms: nausea, vomiting, cold chills, heat waves.
On the night of 24 August, a man believed himself to be an aeroplane and died by jumping from a second-story window, and an 11-year-old boy tried to strangle his mother.
Another family shared a loaf of Briand's bread among five of its seven members, the others preferring biscottes, with only the five falling ill. On the morning of the 20th, the health service, the prefecture, the prosecutor of the Republic and the police were notified.
[8] The police investigation would eventually center on the second of three batches of bread made at Briand's bakery on the day of 16 August.
The investigation led police to interrogate many of the town's residents, who gave inconsistent ratings of Briand's tainted batch.
[8]: 319 On the 23rd of August, a judge of inquiry opened a formal investigation, and tasked commissaire Georges Sigaud with finding the cause of the mass poisoning event.
Both the salt and the yeast used by Briand were sourced from the same suppliers as all other bakers in the region, and subsequent testing of the supplies found no toxicity.
This was unusual, given that owing to a shortage of wheat, the grain control board, ONIC, had mandated that rye flour be mixed in.
However, in the Vienne department, rye of good quality was often more expensive than wheat, and accordingly, bean flour was authorised by ONIC as a replacement.
[8]: 471 Shortly after the incident, in September 1951, Dr. Gabbai and colleagues published a paper in the British Medical Journal declaring that "the outbreak of poisoning" was produced by ergot fungus.
[10] According to reports at the time, the flour had been contaminated by the fungus Claviceps purpurea (ergot), which produces alkaloids that are structurally similar to the hallucinogenic drug lysergic acid diethylamide (LSD).
[11] This type of contamination was considered owing to the presence of fluorescent stains on the outside of some used empty flour bags returned to the distributor.
Panogen was sold containing a red colorant as a safety measure, to ensure that seeds coated with it would be used only for planting.
[12] In 1982, a French researcher suggested Aspergillus fumigatus, a toxic fungus produced in grain silos, as a potential culprit.
[13] Historian Steven Kaplan's 2008 book, Le Pain Maudit states that the poisoning might have been caused by nitrogen trichloride used to artificially (and illegally) bleach flour.
[8][14] In his 2009 book, A Terrible Mistake, author and investigative journalist Hank P. Albarelli Jr writes that the Special Operations Division of the Central Intelligence Agency (CIA) tested the use of LSD on the population of Pont-Saint-Esprit as part of its MKNAOMI biological warfare program, in a field test called "Project SPAN".
Albarelli's view was reported widely after the book's publication, including by The Daily Telegraph, France 24 and BBC News.