Crews have recovered six of nine workers missing and presumed to be dead after a chemical tank rupture in Washington state, officials said Thursday.

The total death toll is 11, including three who are still missing. It’s one of the deadliest U.S. workplace accidents in recent decades.

Officials say the paper mill tank ruptured and spilled more than 500,000 gallons of a highly destructive chemical mixture used in paper manufacturing at the Nippon Dynawave Packaging Co. in Longview, a city along the Columbia River.

Among the 11 victims were a grandfather who was always willing to help anyone and a young husband described as selfless and caring, according to friends who organized fundraisers for the victims’ families.

Fire officials said Wednesday that the recovery of the missing would be slow and deliberate because of the dangers posed by the remaining chemicals.

Authorities said the cause of the disaster is still under investigation. They have not released the names of those who were killed, but friends and relatives had begun confirming their names and posting online fundraisers to support their families.

The tank failure also injured eight people, including a firefighter. Some suffered burns or inhalation injuries, authorities said.

The mill’s Japanese parent company, Nippon Paper Group, said in a statement Wednesday that it was offering its “deepest condolences and heartfelt sympathies to the bereaved families.”

Authorities said Wednesday that the spill hadn’t contaminated the air and drinking water in and around Longview, a city of about 40,000 people near Washington’s border with Oregon. The community, which was founded at the confluence of the Cowlitz and Columbia rivers by a Kansas City timber baron in the 1920s, has deep ties to paper and lumber industries. Generations of families have worked in the mills, and many residents who spoke with The Associated Press had family members or friends connected to the Nippon Dynawave plant.

Crews were working to flush water from ditches near the plant and dilute it before pumping it into the Columbia.