"These severe ice conditions have not occurred on the Great Lakes for more than three decades," the letter states.
Treacherous ice covering much of Lake Superior has prevented freighter ships from hauling iron ore — an essential ingredient in steelmaking — from Minnesota's Iron Range to northwestern Indiana steel mills.
The Great Lakes' ice cover hit its highest point in 35 years this winter, and Lake Superior remains more than 80 percent frozen with a 40-inch-thick shelf ice and stacks of ice chunks reaching up to 14 feet tall. U.S. Steel said it is working closely with United States and Canadian governmental authorities "to expedite and obtain priority passage of our raw materials vessels" on the lakes.
The company has informed its customers that it might not be able to fill their steel orders but is trying to find alternative supply paths.
The Gary Works complex is the nation's largest steel mill, stretching seven miles along Lake Michigan and capable of producing 7.5 million net tons of steel a year.
More than 5,800 employees who work at the mill continue reporting to work despite the temporary suspension of steelmaking operations.