At least some of the hundreds of fish found dead downstream of Ashland's sewage treatment plant in late June apparently were exposed to lethal amounts of chlorine.
Investigators say they can't pinpoint the source of the chemical, but they don't have any suspected source other than the plant on Ashland Creek.
A city official says an investigation hasn't uncovered how a toxic release could have come from the plant.
The death toll was 656 wild juvenile steelhead.
The Medford Mail Tribune reported that tests showed some were exposed to up to 10 times the amount of chlorine that the state Department of Environmental Quality considers toxic to fish.