An analysis of xenon isotopes created four days after the Chernobyl explosion of 1986 in Kiev, Ukraine is challenging our understanding of what happened on that catastrophic day.

It’s long been accepted that the explosion, which killed 30 people, was a steam one—not nuclear. A group of scientists from the Swedish Defense Research Agency, Swedish Meteorological and Hydrological Institute, and Stockholm University says that an assessment of the isotopes shows that they’re the products of nuclear fission—meaning they could have originated from a nuclear explosion.