Mario Corbeil was 15 when a routine motorcycle outing in Longueuil, Quebec, on the evening of May 20, 1975, ended in a double homicide. Earlier that night his parents had given him a small red Kawasaki; at about 8:00 to 8:15 p.m. he offered a ride to neighbor and friend Diane Déry. The pair rode into wooded trails near Boulevard Rolland-Therrien and did not return.

Their families searched through the night and found both bodies about 7:20 a.m. on May 21 on vacant ground near the southern end of the boulevard extension. Both victims had been shot; Mario suffered multiple .22-caliber gunshot wounds (reports describe six wounds and medical notes list projectiles in his head and upper left buttock) while Diane was struck twice. Ballistics pointed to a .22 firearm, with later work identifying a semiautomatic Cooey Sure-Shot rifle as the suspected type.

Investigators focused on local youths who used the woods and had practiced with .22 rifles; several such firearms were seized and examined but none produced evidence sufficient for charges. Decades of reporting later highlighted a former school acquaintance who owned a Cooey rifle, but no court has found anyone responsible. The Sûreté du Québec still lists the killings as unsolved.

Did anyone who knew Mario or Diane, or who spent time in the Rolland-Therrien woods in 1975, ever notice people practicing with rifles or behaving unusually that night?