Officials had previously confirmed two fatalities. At least eight other people were injured, some critically.
Some of those who were injured suffered burns or inhalation injuries, authorities said.
"We're bracing ourselves for this being the deadliest industrial tragedy in modern Washington state history," Washington Governor Bob Ferguson told a news conference.
Authorities said they don't yet know what caused a 3.4 million litre vat at the Nippon Dynawave Packaging facility to implode early on Tuesday, local time.
The container held "white liquor" - a chemical solution of sodium hydroxide and sodium sulphide used in making paper pulp that can cause severe burns on skin.
Longview Fire Department Battalion Chief Matt Amos told the news conference 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 they had searched the area that was searchable.
"As of this morning, we declared this incident a transition from rescue to recovery," he said on Wednesday, local time.
Authorities at the press conference said 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 2-2.1 million litres 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 about a dozen carp had died.
Nippon Paper Group in a statement said it was offering its "deepest condolences and heartfelt sympathies to the bereaved families".
Nippon Paper Industries 3863.T, Japan's second-biggest paper manufacturer by sales, acquired the Longview plant from Seattle-based timber company Weyerhaeuser WY.N for $US225 million ($A315 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 on Tuesday 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.
With AP
