Researchers from Stockholm University and Vanderbilt University claim to have found evidence linking mysterious sky flashes from the 1940s and 1950s to nuclear weapons testing sites in the U.S.

The flashes, described as “transient star-like objects,” appeared in historical Palomar Observatory photos taken before the first satellites were launched, ruling out human-made orbital objects.

Analysis showed the flashes were 68% more likely to occur the day after a nuclear test and increased alongside UFO sighting reports, USA Today has reported.

While the scientists avoided concluding the phenomena were extraterrestrial, the findings renew speculation that UFO activity may be connected to nuclear events.