Two friends on a morning mountain bike ride 30 miles (48 kilometres) east of Seattle were attacked by a cougar, killing one of the men and leaving the second hospitalised.
The cougar was later found up a tree near the dead man's body, where agents for the state's Fish and Wildlife police shot and killed it hours after the Saturday attack, the Seattle Times reported.
The names of neither man were immediately released.
The 31-year-old survivor said he and his friend were mountain biking in the Lake Hancock forest area at about 11am when they realised they were being chased by the big cat.
"So they stopped and they made a lot of noise, said Capt. Alan Myers of the state department of Fish and Wildlife, ''which is exactly what we counsel people to do.
"The two victims then took a minute and were catching their breath about this amazing, incredibly scary event that just occurred and suddenly the victim (the survivor) was attacked again by this cougar.
"It latched onto his head. He said he had his whole entire head in the jaws of this animal and was being shaken around very, very horribly."
Here’s a photo of the cougar that attacked two men on a mountain bike ride near North Bend today. One man died. Another man is at Harborview Medical Center tonight @komonews#komonews#Cougarattackpic.twitter.com/gVQSLL3CwB— Tammy Mutasa (@TammyKOMO) May 20, 2018
The second victim turned and started to run away, and the cougar went after him and killed the 32-year-old.
The injured man rode for about 2 miles (3.2 kilometres) before getting mobile coverage and calling emergency services.
Patient status update: The 31-year-old male who arrived at Harborview this afternoon as a result of a mountain lion attack near North Bend is being treated in the Emergency Dept. He is currently awake and alert and in serious condition.— Harborview Seattle (@harborviewmc) May 19, 2018
Sgt. Ryan Abbott said when rescuers arrived it took them about 30 minutes to locate the second victim, who was dead with the cougar standing on top of him.
"The deputies shot at him and spooked him, and he ran off," he said.
It took several hours before authorities found the cougar up a tree 45 to 182 metres away and killed it. While the cougar was being hunted, rescuers had to wait to retrieve the dead man's body.
Rich Beausoleil, the state's bear and cougar specialist, said it was only the second fatality in the state in the last 94 years. "But it's one too many," he said.
Cougars, also known as mountain lions and pumas, are a protected species, the Times reported. Each year, the state allows 250 cougars to be hunted and killed in 50 designated zones.