Tony Bjerkness yelled for help after his snowmobile plunged through thin ice on Serpent Lake early Monday, and a Crosby, Minn., police officer heard his voice.

"Then all went silent," said Sgt. Scott Goddard of the Crow Wing County Sheriff's Office.

Hours later, a dive team from the Sheriff's Office pulled Bjerkness' body from 45 feet of water a few hundred yards from shore. The 34-year-old man from nearby Ironton was pronounced dead at the scene.

His death is a reminder for anybody venturing onto a lake that ice conditions can change rapidly from place to place, even on the same body of water, Goddard said. On Monday, while ice on Serpent Lake was close to 6 inches thick close to the shoreline, the area where Bjerkness went through had open water Sunday and had just frozen over hours before he took his machine onto the lake around 5:30 a.m.

"This is an example of the extremes that we have," Goddard said. "It's not worth taking the risk to go out."

The Minnesota Department of Natural Resources recommends that ice should be 4 inches thick to support a person and 5 to 7 inches for snowmobiles or ATVs. More than 8 to 12 inches are necessary for cars and small pickups and 12 to 15 inches for a medium-size vehicle. Even at that thickness, "ice is never 100 percent safe," Goddard said.

A police officer from Crosby had stepped out of the station and heard Bjerkness cry for help. The officer noticed something moving on the lake in the darkness. He determined the yelling was coming from the lake.

First responders located snowmobile tracks leading onto the lake and eventually to the spot where Bjerkness broke through. Divers battled "tough conditions," including strong winds and temperatures that caused equipment to freeze up during the search. Bjerkness was pulled from the water around 1:30 p.m., Goddard said.