LONGVIEW, Wash. (AP) — 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.

WATCH: 11 presumed dead after Washington paper mill tank implosion

Officials say the paper mill tank ruptured and spilled more than 500,000 gallons (1.9 million liters) 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.

Gilbert Bernal, a grandfather who was an electrician at the plant, was the first confirmed death, said his friend Todd Cornwell.

"He was one of the most genuinely good people that you've ever met. He would give you the shirt off his back if you needed it. He was always there willing to help in whatever needed to be done," Cornwell said.

CJ Doran, who was 26, was among those presumed dead, according to a GoFundMe post verified by the crowdfunding site.

He was a husband who was "the spiritual leader of their family, the joy of their home, and the family provider," the post said.

Other victims included John Forsberg, described as a father to two young children; Jared Ammons, who had two children and another on the way; and Braydon Finkas, an electrician at the plant who, along with his longtime partner, Kaitlyn Kincaid, took in exchange students and others in need at their home in Cathlamet, according to their friend Rex Czuba.

Finkas was always willing to help someone cut hay or to buy a beer for a new face in their small town, he said.

"He was a really big part of the town," Czuba said. "He really jumped in and became a part of the community so quickly."

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 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.

Some contamination has reached the river, but the U.S. Environmental Protection Agency said it has had no noticeable effect.

Johnson reported from Seattle. Associated Press Martha Bellisle in Seattle, Kathy McCormack in Concord, New Hampshire, and John Seewer in Toledo, Ohio, contributed.