We don't know whether it was from a company or somebody was dumping a chemical, but something foreign went into the river. The free-flowing river had already washed away any clues of what inflicted the kill. By the next morning, fish had returned to the area where the kill occurred, the live fish hovering above the dead. Banks said that scientists took this as a sign that a quick pulse of toxin, and not a chronic water-quality problem, caused the damage. The only clue from the site is a whiff of chlorine reported by a Department of Natural Resources police officer who first surveyed the scene.
Chlorine is highly toxic to fish and life in general.
According to Banks, a single bottle of household bleach spilled into the river would have been enough to do the job. This is a significant kill.