A huge tank holding tens of thousands of litres of a highly caustic substance imploded on Tuesday at the plant in Longview, Washington state, sparking a major operation.

“We can confirm that six of the nine individuals have been recovered,” Brad Hannig, fire chief of Longview fire department, told a news conference.

“We continue working with the coroner to notify families,” Hannig said.

Two people were previously confirmed dead on Wednesday, bringing the death toll to eight.

The three others still missing and feared dead.

The accident at the Nippon Dynawave Packaging Company happened during an early morning shift change when a 3.4 million-liter tank containing a large quantity of a substance called white liquor ruptured.

White liquor is a highly alkaline solution containing sodium hydroxide and sodium sulfide that is used to break down wood chips and create the pulp from which paper is made.

Rescuers are working in an “active and hazardous recovery environment” to find the remaining victims, Hannig said.

Officers sought to reassure the public that drinking water and air around the site were not contaminated.

“Longview’s water is safe,” Chris Collins, public works director for the City of Longview told the news conference.

Brooks Stanfield, a federal on-site officer from the Environmental Protection Agency said that monitoring had not detected hydrogen sulfide or any other air contaminants that may have been a cause for concern.

Nippon Dynawave Packaging, which is a subsidiary of Japan’s Nippon Paper Group, says on its website that it produces eight billion single-serve containers every year, supplying customers in North America, Asia, and around the world.