Deputies said the man, who is from the area, left the park on the east side of the lake at around 8 p.m. Saturday and was swimming toward a private dock about 300 yards away with his younger cousin. He made sure his cousin got to the dock, but he somehow slipper under water, deputies said.
Deputies say the man's cousin called for help right away.
A water rescue team arrived and began to search the lake. They found the man about 15 feet from the edge of the dock, deputies said. He was in about 10-12 feet of water.
Rescuers pulled the man out. First responders started to perform CPR on him, but he did not survive.
Deputies said the man had been under water for about 35 minutes.
"No matter how strong a person thinks they're a swimmer, I've pulled many people from the bottom of lakes and rivers where the family said they were an expert swimmer. Water is unforgiving. It doesn't care who you are, what color you are, it will hurt you if you don't respect it," said King County Sheriff's Deputy Chris Bedker.
Deputies said Fivemile Lake is a spring-fed lake, so it's fairly cold. That can sometimes create a shock to the body, deputies said.
The park doesn't have any lifeguards on duty.
The man was not wearing a life jacket, deputies said. He had been drinking, they added. But investigators don't know if that played a role in what happened.
The man's name has not be released.