KIRKLAND, Wash. -- Police are trying to determine whether a troubled swimmer went under in Lake Washington on Monday night.

Witnesses reported hearing a man's cries for help shortly after seeing a person in the water near Marina Park around 8:10 p.m. The swimmer was about 100 feet out in the water near a buoy, witnesses said.

"My friend, Jay, started taking off his jacket, and thought he was going to dive in the water until I reminded him how cold it was," said witness Charlie Whetstein.

"The third cry for help was 'help me!' and it was garbled, as if he was really in trouble. We didn't hear any splashing, and we didn't see a boat or canoe or anything."

Police have found a person's shoes and wallet on the shore near a gazebo.

A police helicopter flew over the water but did not spot the swimmer. Divers searched for several around the buoy where the swimmer is believed to have gone under, but did not find any sign of him.

Investigators said the situation is still not entirely clear.

"We're not sure," said spokesman Nick Sebert. "We're following up on all leads. We don't know if this is a suspicious death, an attempted suicide or just an accident. We'll follow up on those leads as we get them."

Divers did not plan to search through the night. They said they planned to continue in the morning at which time the search would become a recovery effort.