LAPORTE, Ind. — Authorities worked Monday to reach more than 70 motorists trapped in their cars in biting temperatures in snow-covered northwest Indiana.

LaPorte County sheriff’s Deputy Andy Hynek said officials didn’t know exactly how many people were stranded, but some had been stuck for as long as 12 hours.

The heavy lake-effect snow in northern Indiana was part of a storm that has been crawling across the Midwest since Friday night. At least 11 deaths have been attributed to the storm, which dumped nearly 2 feet of snow in parts of Minnesota and Wisconsin before moving into Michigan and Indiana. On Monday it stretched farther east, with snow in parts of Ohio, Pennsylvania and New York.

Northwest Indiana was hardest hit Monday, with up to 16 inches of snow in some areas around LaPorte. Lake-effect snow develops when cold air rushes over the warmer water in Lake Michigan.

About 70 vehicles were trapped by snowdrifts Monday morning on a section of state Route 2 in the Valparaiso area. Police said the motorists were found warm and safe in their vehicles.

Crews were using front-end loaders to remove drifts on U.S. Route 30, where other drivers were trapped overnight, state highway department spokesman Jim Pinkerton said. Sections of Route 2 and U.S. Route 30 were closed, and with winds of up to 30 mph, LaPorte and Porter counties issued emergency orders telling drivers to stay off county roads as well.

“As soon as the plows go through an area, the wind is blowing fresh snow right back into the roads,” Pinkerton said. “It is just really difficult for us to keep up against that wind and snow.”