At least five people died, more than 30 were hospitalised and almost 300 taken ill in the unexplained outbreak in Pont-Saint-Esprit, a town in the Gard, south-east France.
On 17 August 1951, local doctors' surgeries were filled with people complaining of sudden nausea, diarrhea and insomnia. Hours later, many of them were in the grip of terrifying and often violent hallucinations.
One girl believed she was being attacked by tigers. A man tried to drown himself after seeing snakes in his stomach. Another flung himself from a window, believing he was an aeroplane. And an 11-year-old boy tried to strangle his grandmother.
An investigation at the time determined that the town's main bakery was to blame.
Scientists suspected that the baker's flour had been accidentally contaminated with a naturally occuring hallucinogenic substance - most likely ergot, a type of psychedelic mould.
This explanation was never considered fully satisfactory, however. A US laboratory that asked volunteers to ingest ergot-dosed bread found that none of its subjects presented similar symptoms to the residents of Pont-Saint-Esprit.
Almost 60 years later, new evidence has been uncovered that suggests more sinister forces were at work.
According to investigative journalist Hank P Albarelli Jr, the CIA and US Army deliberately contaminated Pont-Saint-Esprit with LSD as part of Cold War-era research into mass mind control.
Albarelli claims to have uncovered CIA documents that refer to "the secret of Pont-Saint-Esprit" and explain that the symptoms were called not by ergot but by diethylamide, one of the main substances - the D - in LSD.
The documents transcribe conversations between CIA agents and officials from Sandoz Pharmaceutical Company, the Swiss laboratory where LSD was first developed - and where the scientists charged with investigating the Pont-Saint-Esprit outbreak were employed.
Albarelli says he also spoke to scientists at the US Army's Special Operations Division at Fort Detrick, Maryland, who told him that, in their quest to discover whether LSD could be used as a weapon, agents sprayed the drug into the air of Pont-Saint-Esprit and planted it in local food products.
But his most compelling evidence is a White House document that was sent to members of the Rockefeller Commission, a panel formed in 1975 to investigate alleged CIA abuses. This document mentions several French nationals who were covertly employed by the CIA, and refers directly to the "Pont St. Esprit [sic] incident".
None of Albarelli's sources would confirm whether France's secret services were aware of the alleged experiment, he writes in his book about the affair.
It's not the first conspiracy theory to be advanced about the strange events in Pont-Saint-Esprit.
"At the time, people speculated whether it was an experiment designed to control a popular revolt," recalls one Pont-Saint-Esprit resident, Charles Granjhon, now 71.
He still doesn't feel he knows the truth about what happened, he told Les Inrockuptibles magazine.
"I almost snuffed it. I'd like to know why."