The U.S. Chemical Safety Board intends to investigate the cause of the leak that allowed 7,500 gallons of coal-processing chemicals to spill into the Elk River in West Virginia.
The chemicals flowed from a tank owned by privately held Freedom Industries.
The leak took place upstream from a treatment plant operated by American Water (AWK), which said it would offer rebates to customers for their inability to use the water. Up to 300,000 have been affected.
Tests indicate that the contamination is now low enough that it may soon be possible to clear the water for consumption, although that approval may still be days away.