Authorities suspect anhydrous ammonia killed thousands of fish in a stretch of southeast Nebraska's Little Blue River.
Thayer County Sheriff David Lee reports several calls about the fish, which were spotted in Hebron and downstream for about eight miles.
Spokesman Brian McManus of the Nebraska Department of Environmental says officials are still checking but it's believed the culprit was the ammonia, a common farm fertilizer.
He says state officials don't know yet whether the spill was an accidental or deliberate act.
Corrective action includes fines ranging into the thousands of dollars and paying to restock fish into the river.