(AP) — Authorities are perplexed as to what may have caused several hundred fish to die in eastern Nebraska.

Officials at David City Park are warning visitors to avoid contact with its lake water while local and state officials investigate the dead fish that started popping up in late April.

Park Supervisor Scott Bales says most of the fish are bluegill, which are the most populous fish in the water.

Officials with the Nebraska Department of Environmental Quality have tested the water and they appear to have plenty of oxygen.

Program Specialist David Bubb says one possibility is ammonia, but he couldn’t pinpoint the source. He says water upstream from David City Park does not appear to be affected.