Holes in a tank's floor and roof likely helped cause a January chemical spill that contaminated West Virginia's biggest drinking water supply for days, federal investigators said.
In a report released Wednesday, the Chemical Safety Board also found that the leaky Freedom Industries tank wasn't the only shoddy one.
Another Freedom tank that held the same spilled chemical had a similar hole in its floor. The board detailed three crude MCHM tanks with pitting, holes or other damage.
"The holes that we found in the ... main tank wasn't the only hole," CSB investigator Johnnie Banks told reporters after a public hearing in Charleston. "So we're going to continue our investigation along those lines. We've gathered some samples of metals and other materials to place those under closer scrutiny and make a determination on the possible failure mode for the tanks."
The board said corrosion from water pooling inside the leaky tank likely caused the holes. The substances traveled through soil, gravel and water systems under the Freedom site before hitting the Elk River.
In a few other spots, the water started corroding away the tank's floor, but didn't penetrate it.
The same tank had holes in its roof, which likely let corrosion-inducing water enter the tank, the board said.
The board couldn't find records of tank inspections before the January spill. It is unclear how often or rigorously inspections occurred.
Banks said several months after the leak, inspectors went inside the tanks, which had been emptied of all materials, and it was determined where corrosion was most prevalent.
An inspection done in late 2013 as part of the terminal's sale to Freedom Industries "didn't require the emptying of the tank, which is really the key difference and significance between the earlier inspection and the later ones," Banks said.
Officials say the Freedom tanks fell into a regulatory gray area. West Virginia lawmakers responded by instituting a slew of new registration and inspection requirements for aboveground storage tanks, like those at Freedom.
The January spill in Charleston let coal-cleaning chemicals seep into the river and eventually enter the West Virginia American Water's intake 1.5 miles downstream.
Officials deemed tap water unsafe for 300,000 people for four to 10 days, except for fighting fires and flushing toilets.
Chemical investigators arrived at Freedom less than two days after the spill. They have said it will about a year total to release a full report on the spill.
The board plans to study several other areas, including: a metallurgical examination; computer model of the spill; consideration of public health impacts; aboveground tank regulations; the tank's location upstream from the water intake; evaluation of water plant intake systems; and emergency planning and notification.
The offices of U.S. Attorney Booth Goodwin and Attorney General Patrick Morrisey are also investigating the spill.
Earlier this month, the federal Occupational Safety and Health Administration fined Freedom $11,000 for two workplace violations.
Associated Press writer John Raby in Charleston contributed to this report.