A spate of accidents involving snowmobiles and full-size vehicles has made this winter the state’s deadliest on lake and river ice in more than five years, the Minnesota Department of Natural Resources said Friday.

Five people have died and a sixth person is missing and presumed drowned with more than a month remaining in what state officials consider the winter recreation season. All the fatalities involved motorized vehicles crashing into open water or breaking through thin ice where currents were faster.

“I think it has a lot to do with people who don’t think it could happen to them,” said Kara Owens, a DNR boat and water safety specialist. “Ice is always unpredictable. We have had a colder winter but there is still open water.”

The last big deadly season was in 2006-07, when eight people died in ice-related incidents.

Last Saturday, 22-year-old Billy Meister of Stillwater drowned in the St. Croix River near Bayport when he drove his snowmobile into open water.

Just hours later, a snowmobiler farther north in Pine County plunged through the ice on the St. Croix and disappeared.

Sheriff Robin Cole said Friday he didn’t know when he would name the missing snowmobiler: “At the time he’s declared dead or we find him, either way. We don’t have a death certificate on him, so he’s considered a missing person,” Cole said.

The search remains suspended because of dangerous ice conditions, the sheriff said.

Despite impending warmer weather and weakening ice, many people continue to drive on “ice roads” that cross the St. Croix, and recreational snowmobile traffic has been heavy on the river, said Dan Starry, chief deputy at the Washington County Sheriff’s Office.

“Know the area you’re snowmobiling,” Starry said. “Certainly go with more than one person. You have to be aware of the ever-changing conditions with the weather that is coming up.”

DNR enforcement officers who patrol Washington County lakes and the St. Croix River said Friday they haven’t encountered other instances of snowmobilers at peril but said people should use good judgment, particularly as winter wanes.

“Anytime you venture off that main trail, there’s always a higher percent of something happening,” said Officer Scott Arntzen, who patrols Stillwater north to Scandia.

Officer Amber Peterson said ice is 20 inches thick over much of the St. Croix and area lakes but open water can occur wherever the current is faster or incoming water is warmer.

“It’s just a matter of people driving vehicles where they’re not supposed to,” Peterson said of recent deaths.

Earlier this month, Harland Dietrich, 31, and his grandmother Mary Ann Haram, 87, were trapped underwater and died when a car Dietrich was driving broke through thin ice on a Lake Minnetonka channel. Two weeks earlier, 8-month-old Tabitha Markle died when her family’s vehicle broke through another channel on the same lake. Her father, who was driving and intoxicated, was charged with felony vehicular homicide.

Last year, four people died in ice accidents. The same number died the previous winter. In the winter of 2009-10, only one death was recorded.

“There could be several reasons why so many people have died this year,” Owens said. “It could be there are more people out on the ice because we have had a cooler winter and more snow.”

As the winter winds down and Minnesotans enter the last weekend in February, Owens has an urgent message for winter enthusiasts: “The bottom line is it’s crucial that people do not let their guard down and recognize ice is never 100 percent safe.”

Dark houses, fish houses and portables must be off the ice of inland waters no later than midnight on March 4 in the southern two-thirds of Minnesota and March 18 in the northern third. Enforcement action will be taken if shelters are left after the deadlines, the DNR said Friday.

The March 4 removal deadline applies to waters south of a line starting at the Minnesota-North Dakota border near Moorhead along Hwy. 10, extending east along Hwy. 34 to Hwy. 200, east along Hwy. 200 to Hwy. 2, and east along Hwy. 2 to the Minnesota-Wisconsin border near Duluth. The March 18 deadline applies to waters north of that line.