The brothers were working on the exhaust manifold of an antique car in the closed garage at one of the men's Chehalis home, deputies said. The other brother was visiting from Rainier.
When the wife of one of the victims returned home from a neighbor's home, she discovered the two men unconscious in the garage and called 911.
"She was pretty upset," said family friend Barbara Boohm, "And she said 'How many times have I told them to leave that garage door open?' And I thought it was kid of weird because yes they keep it open."
Medics arrived to find high levels of carbon monoxide in the garage and rushed both victims to area hospitals. One brother was taken to Centralia Providence Hospital where he was later pronounced dead. He was identified by the Lewis County coroner as William Schofield.
The other brother, Walter Schofield, was taken to Harborview Medical Center in Seattle where he later died Monday morning.
"These brothers were simply working on a car together and ended up dying from a silent killer," Lewis County Sheriff Steve Mansfield said in a statement. "Educate yourself about the dangers of carbon monoxide poisoning and keep you, your friends and family safe."
Sheriff's investigators believe they fired up the engine not thinking about the danger of being in an enclosed space. "I mean they probably intended to start it up, I don't know, said Lewis County Sheriff Steve Mansfield, "But the bottom line is it overcame them so quickly and by the time the wife of one of them came back it was already too late."
Investigators believe this was purely accidental, not intentional.
"There's no indication to show us that they did this to themselves or one did it to the other one at all," Mansfield said. "None whatsoever."
Friends say the twins were too full of life and had too much to live for. They were about to embark on a trip to Japan in a few weeks.
"They were 80 years old. They continued to do what they loved to do," Boohm said. "They had more energy. Way more than I've ever had." They were described as warm and loving.