One person was killed and nine others remained unaccounted for after a tank ruptured at a paper mill in Longview, Wash., on Tuesday, releasing about 500,000 gallons of a highly caustic chemical, the authorities said.

Nine other people were injured, including one firefighter, officials said on Tuesday evening. The mill is operated by the Nippon Dynawave Packaging Company.

Efforts to recover bodies had slowed because the tank remained unstable, with about 90,000 gallons of a mixture known as “white liquor” still at risk of leaking out, Chief Scott Goldstein of Cowlitz 2 Fire and Rescue said at a news conference on Tuesday evening. There was also a risk that the tank could collapse, he said.

“White liquor” is a solution of sodium sulfide and sodium hydroxide used to make pulp from wood chips in a process involving high pressure and temperatures. It can cause second- or third-degree burns on contact with skin and can also be dangerous if inhaled, said Matt Amos, a shift battalion chief with the Longview Fire Department.

It was unclear what had caused the tank to rupture and whether there had been an explosion or implosion, where a structure collapses inward, officials said on Tuesday evening.

They said there was no direct risk to the surrounding community, but urged residents to avoid the area and nearby ditches that may have been contaminated by the chemical.

Eight employees and one firefighter were transported to hospitals with injuries ranging from minor to severe, officials said. The firefighter had been treated and released from the hospital, Mr. Amos said.

The rupture at the Nippon plant, which is about 45 miles northwest of Portland, Ore., and makes wood pulp and paper board for packaging and cartons, occurred shortly before 7:15 a.m. local time, the authorities said.

In a joint statement, Nippon and emergency authorities in Longview said a tank containing chemicals ruptured, causing “multiple critical injuries.”

The fire department initially described the episode as a “major chemical explosion,” but said in a subsequent statement that the cause was an “implosion involving a vat of chemical treatment product” at the plant.

A person who answered the phone at the Nippon mill declined to comment.

Experts said that a fire at the Nippon mill in July 2023 caused air quality readings to reach unhealthy levels in Portland. Several wood chip piles, each of which is about the size of a football field, caught fire, a company official said at the time.

The 2023 fire was ruled an accident, but its cause remained unknown, according to an official report by Cowlitz 2 Fire and Rescue cited by The Columbian newspaper.

Firefighters also fought a blaze at a locomotive repair shop on the Nippon property in 2025. No injuries were reported in the 2023 or 2025 fires.