A chemical storage tank at a Washington state paper mill had been declared “not fit for continued service” roughly 10 months before it ruptured in May, killing 11 workers in one of the deadliest U.S. workplace accidents in recent years, according to a federal report released Friday.

The U.S. Chemical Safety Board’s latest findings concerning the May 26 tank failure in Longview are part of an investigation that remains underway. The rupture sent a massive volume of caustic liquid through the facility with enough force to blow out walls. Investigators said they have not yet determined exactly what caused the tank to fail.

According to the board, the inspection contractor recommended that Nippon Dynawave Packaging Company conduct internal examinations of the tank and make repairs to its shell. No such work was carried out between the contractor’s July 2025 inspection and the May 2026 disaster.

A request for comment was sent to Nippon Dynawave. An individual who answered the telephone at the company’s Longview facility declined to comment, while a separate message seeking a response was left with the company.

During an external inspection in July 2025, the contractor discovered “significant thinning” in the lower sections of the tank, with portions falling below the minimum thickness considered safe, the federal report said. That minimum threshold is used to determine whether a tank can safely withstand the pressure created by its contents while also supporting its own weight.

The same contractor returned to examine the tank’s exterior in October 2025 and again in February 2026. Both inspections concluded that “large portions of the tank remained thinner than the calculated minimum safe wall thickness.”

“The company also did not remove the tank from operation or derate the tank for reduced service,” the board said.

The May rupture released an estimated 900,000 gallons (3.4 million liters) of white liquor, a highly caustic chemical mixture used in the papermaking process to break down wood. The substance can cause severe chemical burns. Eleven workers died, while several other people and a firefighter were injured.

The July 2025 inspection documentation included in the federal update classified the situation as “Priority 1: Not fit for continued service unless corrected,” and warned of a “high likelihood/consequence of failure (or) critical code/standard deficiency.”

“Owner should plan to have an internal inspection performed in order to determine the extent of the required repairs. It’s likely that the entire shell course should be replaced,” the inspection excerpt says.

The inspection was performed by Applied Technical Services LLC. The federal report did not explain why the contractor had been hired to conduct the examinations.

The tank failed only minutes after workers began arriving at the mill at about 7 a.m., according to investigators.

The sequence of events began several hours earlier. At approximately 2 a.m., a vessel known as a digestor was taken offline so repairs could be performed. That stopped the normal transfer of white liquor from the tank into the digestor, but the chemical continued flowing into the tank. By about 6:20 a.m., the tank was nearing its capacity. It ruptured less than an hour later.

The released chemical first inundated the courtyard, crushing vehicles and damaging part of a nearby building. It then spread into lower levels of surrounding buildings, where employees had gathered for morning meetings.

Eight workers inside the electrical maintenance shop and another worker in the courtyard were killed. Five employees and a foreman in the mechanical maintenance shop managed to escape, but two others in that area died after being exposed to the chemical. Two of those who survived suffered serious chemical burns.

The company contacted its medical emergency response team after the incident but failed to tell responders that white liquor had been released, the report said. One emergency team member entered a pool of the chemical while attempting to rescue a worker, believing the liquid was water. The responder suffered serious chemical burns.

Earlier in the week, the Washington Department of Ecology notified Nippon Dynawave of 35 environmental violations. The state agency is responsible for overseeing the facility’s air and water permits as well as compliance with environmental waste regulations.

The Chemical Safety Board emphasized that its investigation has not been completed. The agency said its full conclusions and findings will be presented in its final investigative report.