Officials had previously confirmed two fatalities. At least eight other people were injured, ​some critically.

“We’re bracing ourselves for this being the deadliest industrial tragedy in modern Washington state history,” Washington Governor ​Bob Ferguson said at a news conference with local authorities on Wednesday.

Authorities said they don’t yet ⁠know what caused a 900,000-gallon vat at the Nippon Dynawave Packaging facility to implode early Tuesday.

The container held “white ​liquor” - a chemical solution of sodium hydroxide and sodium sulfide used in making paper pulp that can cause severe burns on ​skin.

Longview Fire Department Battalion Chief Matt Amos said at the news conference that the recovery effort would proceed slowly in the “extremely hazardous” environment.

It was unclear where exactly the nine missing people were in the facility, Cowlitz 2 Fire Chief Scott Goldstein said, adding that they had ​searched the area that was searchable.

“As of this morning, we declared this incident a transition from rescue to recovery,” he ​said.

Authorities at the press conference said that testing confirmed that contamination entered the Columbia River.

“Testing of water samples has confirmed contamination entered the ‌Columbia ⁠River during the day yesterday.

Additional evaluations are underway to better understand the scope and extent of that environmental impact,“ Goldstein said.

“At this time, there … continue to be no identified negative health impacts to … the surrounding air quality or the city of Longview’s drinking water system.”

Goldstein said it was fair to say roughly 550,000-570,000 gallons left the tank, while some material remains in ​the tank and is continuing ​to slowly leak.

Officials advised people to ⁠stay away from parts of the area and said that about a dozen carp had died.

Nippon Paper Industries, Japan’s second-biggest paper manufacturer by sales, acquired the Longview plant from Seattle-based timber company ​Weyerhaeuser for $225 million and established the wholly-owned subsidiary Nippon Dynawave Packaging in 2016.

In southern California, ​meanwhile, authorities who ⁠had been monitoring an overheating industrial tank containing highly flammable methyl methacrylate said late Tuesday that the GKN Aerospace facility in Garden Grove no longer posed a risk of explosion.

Thousands of people had been ordered to evacuate their homes in the area on ⁠Thursday after ​the chemical in the tank was found to be overheating.

“After an extensive ​operation, we’re happy to report that all evacuation orders have been lifted and residents have started to return home,” TJ McGovern, interim chief of the Orange ​County Fire Authority, said on Tuesday.