State biologists are trying to unravel a mystery of what killed a herd of elk in northeastern New Mexico.

More than 100 elk found were dead on a ranch about 20 miles north of Las Vegas this week.

Sky News 13 flew over the gruesome discovery on the sprawling 75,000-acre Buena Vista Ranch near Mora.

The elk weren't shot, so the New Mexico Department of Game and Fish is investigating just what caused the deaths.

Their top suspicion: something called Epizootic Hemorrhagic Disease, or EHD. The often-fatal disease is caused by insect bites.

"With EHD, an elk could get a fever," said Game and Fish spokesperson Rachel Shockley. "It's usually a pretty fast illness, and up to eight to 36 hours later the animals go into shock, and then they die."

With elk bow hunting season starting on Sunday, some guided expeditions in the area may be called off.

Biologists are sending tissue samples from the elk and water samples from the area for testing.

If it is EHD, Game and Fish says it's not contagious to humans. The disease is spread from insect bites, not animal to animal.

Game and Fish say no other die-offs of elk have been reported in New Mexico so far this year.

They say hunters should avoid harvesting elk that appear sick and to call and report anything unusual.