Here’s what you’ll learn when your read this story:

• On the night of July 2, 1951, in St. Petersburg, Florida, 67-year-old Mary Reeser died in a fire in her apartment.
• Strangely, the blaze seemed limited solely to the area around her body, leaving her apartment undamaged.
• FBI officials considered the tragedy an example of the “wick effect,” where an external heat source—in this case, a cigarette—started a fire that was then sustained by fat stored in the human body.

One of the most enduring tropes of the detective genre is the locked-room mystery, where a crime or murder takes place in a setting—such as a room no one could have entered or exited—that feels almost impossible. A real-life example of such a case is the strange death of 67-year-old Mary Reeser, which left behind head-scratching details that are as hard to explain as the toughest Holmesian mystery. The case was so puzzling that some attributed it to spontaneous combustion—the pseudo-scientific theory that the human body can, in specific circumstances, burst into flames.

Mary Reeser was a resident at an apartment complex at 1200 Cherry Street in St. Petersburg, Florida. On July 2, 1951, Reeser reportedly told her son, Richard (who was a doctor), that she planned to take at least two sleeping aids known as seconal pills—fast-acting barbituratea that are typically used for insomnia or pre-surgical sedation. Richard didn’t know it at the time, but he would be the last person to see his mother alive.

At 5 a.m. the next day, Pansy Carpenter—the building’s landlady—smelled smoke, but it wasn’t until 8 a.m. that she tried to gain entry to Reeser’s apartment. Because the door handle was too hot, she got help from house painters to open the door, and what they found inside was a horror show. Scorched remains lay heaped in a pile of ash, including pieces of vertebrae and even Mary’s skull, which incredibly had “shrunk to the size of cup,” according to The Tampa Bay Times. Curiously, her left foot remained unburned.

Experts were confounded—for a body to burn to such a degree, it’d need to be heated to roughly 3,000 degrees Fahrenheit for three to four hours. But apart from some melted candles, ashes on the ceiling, a devastated upholstered armchair, and some warped electrical switches, the apartment was relatively unscathed. In fact, some extra-flammable newspapers appeared completely untouched.

St. Petersburg Police Chief J.R. Reichart immediately knew he was out of his depth, and wrote a month after the incident that this was “the most unusual case I’ve seen during my almost 25 years of police work in the City of St. Petersburg.” In a letter to J. Edgar Hoover, head of the Federal Bureau of Investigation at the time, Reichart declared that the mysterious fire “is too puzzling for the small-town force to handle,” and the police sent samples of the apartment’s rug, rubble from its walls, and pieces of the ruined chair to a laboratory in D.C. for analysis. According to the Tampa Bay Times, the feds ruled out lightning strikes or “combustible accelerants,” such as alcohol or gasoline.

So, was this truly a case of spontaneous combustion? Well, also no.

Spontaneous combustion is a medical myth with no supporting evidence that the body contains enough energy to set itself on fire. What’s more likely is an idea known as the “wick effect,” where an external energy source lights a fire and the body’s fat reserves sustain the blaze. And in Reeser’s case, the outside heat source was a frighteningly common one.

“Mary was a great smoker,” Ernestine Reeser, Mary Reeser’s daughter-in-law, told the St. Petersburg Times in 1991. “The cigarette dropped to her lap. Her fat was the fuel that kept her burning. The floor was cement, and the chair was by itself. There was nothing around her to burn.”

Reports stated that Reeser’s nightgown was made of rayon acetate, which is highly flammable. The cigarette likely lit this “wick,” and the body’s isolation from other flammable material (other than the chair) contained the fire to just one spot in the apartment. But while it’s the most likely explanation, not everyone is convinced. Wilton Krogman, an anthropologist from the University of Pennsylvania who was a consultant on the case, said that in such a situation, the skull should have exploded—not shrunk. According to All That’s Interesting, Krogman also reportedly said that he couldn’t “conceive of such complete cremation without more burning in the apartment.”

Unlike so many great detective tales, the incendiary conclusion of this gruesome death still contains some mystery and uncertainty. But what isn’t in question is that Mary Reeser—who became known as the “cinder lady” after her untimely demise—loved needlepoint, entertaining, and her family (she had moved to the area just to be closer to her son and granddaughters).

A fire can burn away a life, but the memories remain.