Officials have struggled to come up with an explanation for the deaths of at least 40 cows over the last two months.
The phenomenon began back in October when a ranch in the town of Meeker, north-west Colorado reported that 18 of its cattle had seemingly dropped dead for no apparent reason.
Initially assuming that wolves were responsible, investigators examined the animals and conducted extensive surveys of the region to look for signs of the predators, but surprisingly found very little evidence to suggest that this is what had happened to the cows.
The mystery soon deepened further still when several other cattle also dropped dead, leading investigators on a hunt for answers that would leave them scratching their heads.
They even considered the possibility that the cows had been infected with a deadly bacteria, but a forensic examination of the remains revealed no evidence of this either.
The phenomena has so far claimed that lives of more than 40 animals.
Travis Black, the area’s Colorado parks and wildlife manager, described the situation as “perplexing”.
“We’re scratching our heads a little bit,” he said. “We don’t know exactly what has occurred up there.”
Efforts to get to the bottom of the mystery remain ongoing.