THERMOPOLIS, Wyo. (AP) — The U.S. Bureau of Land Management says it's investigating the killing of 21 stray horses on federal and state land northwest of Thermopolis.

BLM officials say the horses were found Wednesday. Investigators believe they were killed sometime in the last two weeks.

Wild horses also roam northern Wyoming but BLM spokeswoman Sarah Beckwith said Friday these horses were strays. She says they've been seen running loose only within the past few years.

Beckwith declined to provide additional information including whether the horses may have been shot or poisoned. She says the BLM doesn't want to compromise the investigation by federal, state and local officials by disclosing too much information.

The BLM is offering a reward of up to $2,000 for information leading to the arrest and conviction of those involved.