The death toll from a chemical spill at a paper mill in Washington rose to 11 after rescuers recovered the bodies of all nine people previously reported missing, U.S. authorities said Saturday.

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

"We have recovered the ninth and final missing employee of this incident," Brad Hannig, fire chief of Longview Fire Department, told a news conference.

Rescuers said Wednesday that two people had died while nine others remained missing.

The accident at the Nippon Dynawave Packaging Company happened during an early morning shift change when a 3.4 million-liter (900,000-gallon) 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.

Nippon Dynawave Packaging, which is a subsidiary of Japan's Nippon Paper Group, says on its website that it produces 8 billion single-serve containers every year, supplying customers in North America, Asia and around the world.