“A dull muffled roar gave but an instant’s warning before the top of the tank was blown into the air,” The New York Times wrote in 1919. “Two million gallons of molasses rushed over the streets and converted into a sticky mass the wreckage of several small buildings which had been smashed by the force of the explosion.” “Wagons, carts, and motor trucks were overturned. A number of horses were killed. The street was strewn with debris intermixed with molasses and all traffic was stopped.”

It was January. The place was Boston. And when 2.3 million gallons of molasses burst from a gigantic holding tank in the city’s North End, 21 people were killed and about 150 more were left injured. The wave of syrup — some reports said it was up to 40 feet tall — rushed through the waterfront, destroying buildings, overturning vehicles and pushing a firehouse off its foundation.

For nearly 100 years, no one really knew why the spill was so deadly.

But at a meeting of the American Physical Society this month, a team of scientists and students presented what may be an important piece of the century-old puzzle. They concluded that when a shipment of molasses newly arrived from the Caribbean met the cold winter air of Massachusetts, the conditions were ripe for a calamity to descend upon the city.

By studying the effects of cold weather on molasses, the researchers determined that the disaster was more fatal in the winter than it would have been during a warmer season. The syrup moved quickly enough to cover several blocks within seconds and thickened into a harder goo as it cooled, slowing down the wave but also hindering rescue efforts.

“It’s a ridiculous thing to imagine, a tsunami of molasses drowning the North End of Boston, but then you look at the pictures,” said Shmuel M. Rubinstein, a Harvard professor whose students investigated the disaster.