LONGVIEW, Wash. (AP) — Crews have recovered the remains of six of nine workers who were missing after a massive chemical tank ruptured at at paper mill in Washington state, releasing a flood of caustic liquid capable of causing severe burns and lung injuries, officials said Thursday.

In all, 11 people were killed in the disaster, including the three yet to be recovered, and eight injured. It’s one of the deadliest U.S. workplace accidents in recent decades.

A tank containing more than 500,000 gallons (1.9 million liters) of a chemical mixture used to break down wood for making paper collapsed Tuesday morning at the Nippon Dynawave Packaging Co. in Longview, a city along the Columbia River.

The collapse occurred during a shift change, and the six workers whose remains were recovered had been in an area where they would congregate in the morning as they awaited their assignments for the day, said Matt Amos, Longview fire battalion chief.

Among the 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.

The recovery of the victims has been slow and deliberate, complicated by the dangers posed by the remaining chemicals and other industrial hazards, Amos said.