The prevailing theory for centuries was that the people of Pompeii and Herculaneum, sitting in the shadow of Mount Vesuvius at the moment of its cataclysmic eruption in 79 A.D., were killed by a super-hot cloud of ash blown out of the volcano that caused them to asphyxiate.

But a new theory, looking deep into unusual red-and-black residues on the bones of the Romans famously frozen in time by the blanket of ensuing ash, posits something quicker, but perhaps more gruesome.

The victims were killed instantly by the heat blast as their blood vaporized and their skulls exploded, according to the new analysis published in PLOS ONE.

“The overall evidence including the predominance of life-like stance suggests the occurrence of thermally-induced instant death of the inhabitants in the Vesuvius area up to at least 20 kilometers from the vent,” writes the Italian team.

The sample size was about 80 of the remains, who apparently had gathered in bathhouses along the beach at Herculaneum, the community smaller than Pompeii but closer to the volcano. The ash-preserved forms were excavated by archaeologists in the 1990s (first discovery of the Pompeii and Herculaneum sites was made in the 18th century).

The red incrustations and black marks on the bones were noted. Together they were first assessed by magnifying glass, but then the team tried two methods of in-depth chemical identification.

The team used inductively coupled plasma mass spectrometry of the red-black material, performed using an Agilent 7700 machine, according to the paper.

The Raman microspectroscopy tests came next, looking to determine the iron content of the red stains on the bones, the study adds. (Proteomic analyses were also conducted using liquid chromatography-mass spectrometry on a CHIP MS 6520 QTOF).

The red stains are thought to be iron oxides caused by the near-instantaneous vaporization of the blood and tissue at temperatures of 500 degrees Celsius. The blood traces stuck to the bones, the researchers hypothesize—although they cannot be totally certain of that conclusion.

More concrete are the fractures and charring of the bones, particularly the craniums. The skullcaps were cracked and “exploded,” write the scientists.

“The detection of such iron-containing compounds from the skull and the ash filling the endocranial cavity, coupled with brown coloration of venous sinuses, bone blackening and cracking, strongly suggests a widespread pattern of heat-induced hemorrhage, intracranial pressure increase and bursting, most likely to be the cause of instant death of the inhabitants of Herculaneum,” the investigators conclude.

The victims probably didn’t even have time to realize their fate, according to the postures of the bodies at Herculaneum.

“The lack of voluntary self-protective reaction or agony indicates that any vital activity had to stop within a time shorter than the conscious reaction time, a state known as fulminant shock,” they write. “The widespread occurrence of life-like stance has been found consistent with cadaveric spasm, a rare but diagnostic form of instantaneous muscle stiffening (instant rigor), induced by instate thermal coagulation in victims from pyroclastic currents, which crystallizes the last vital activity prior to death.”