Federal investigators say holes in a storage tank floor likely allowed chemicals to seep out and contaminate West Virginia's biggest drinking water supply in January.
They found a similar hole in the floor of another Freedom Industries tank that held the spilled chemical.
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The Chemical Safety Board says corrosion from water pooling inside the leaky tank likely caused the holes. Holes in the tank's roof likely let corrosion-inducing water into the tank.
The Chemical Safety Board provided the update in a report. Officials also were in Charleston Wednesday talking about the ongoing investigation.
The board said it couldn't find records of tank inspections before the January spill. It is unclear how often or rigorously inspections occurred.
The spill spurred a tap-water ban for 300,000 people for days.