Nearly 50 metal spikes were mysteriously planted in the shallows of an urban lake popular with swimmers, alarming Seattle residents and leading to a police investigation.

"We're horrified, because we can't think of any reason anyone would have placed these except to hurt people," said Dewey Potter, a spokeswoman for the city's Parks Department. "We've never encountered anything like this. It's very bizarre."

Patrick Boltz was wading in Green Lake on Sunday with his wife and 18-month daughter when something cut his foot. He discovered a spike and yanked out 10 spikes in a matter of minutes. He then alerted the parks department.

"It's pretty sickening and depraved," said Boltz, who suffered a small cut between his toes.

The rods were about a quarter-inch thick, and the tips looked as if they had been sharpened, Boltz said.

Parks crews searched the shallows and swimming areas after Boltz notified the department, Potter said. Seattle police divers combed the shores of Green Lake on Thursday but found no additional spikes, Officer Renee Witt said.

The spikes were found close to shore in two spots near an area where kayaks and small sailing craft launch. They were 1 to 2 feet long, Potter said.

People typically don't swim near the boat launch area, but some do, Potter said. Warnings were posted by the parks department around the small crafts center.

Green Lake is in a neighborhood north of downtown Seattle. It's a popular spot for families, and many kids swim there during the hot summer days. Sailing, wind-surfing and pedal boats are also popular. Joggers and bicyclists use a paved trail throughout the year.

Patty Reinholdtsen sat Thursday afternoon next to the area where the spikes were found. Her grandson was just getting back from a sailing lesson. The day before she had seen divers look for the spikes.

"Someone out there tried to hurt somebody," she said. "Somebody is just sick."