The Great Lakes freighter hobbled near Isle Royale over the weekend suffered a 13-foot crack in its steel skin, but the mishap likely wasn't caused by an underwater collision as originally thought, the U.S. Coast Guard said Monday evening.

The nearly 700-foot-long Michipicoten was en route from Two Harbors to Thunder Bay, Canada, on Saturday when it began taking on water. Half of the ship's crew was evacuated, and the vessel then limped on to Thunder Bay.

The Coast Guard first reported the Michipicoten, which was carrying taconite mined on Minnesota's Iron Range, had hit something underwater.

An underwater collision still can't be "100 percent" ruled out, but "there is no evidence to indicate [the ship] struck a submerged object," Lt. Joe Snyder, a spokesman for the U.S. Coast Guard in Sault Ste. Marie, Mich., said Monday evening.

The gash in the ship's hull "is likely due to a stress fracture," he said.

Divers found a 13-foot foot crack below the water line in the "general area" of the bow. The crack varied from one-half inch to one inch wide. A ballast tank flooded due to the crack, as did the "centerline void," an empty space between the hatch floor and the hull, Snyder said.

Temporary patches are being installed on the ship in Thunder Bay to make it seaworthy, Snyder said. The vessel is then expected to sail under its own power to another port where more extensive repairs will be made.

The first reports from the ship indicated an underwater collision, but later conversations with crew members and an initial assessment of the ship indicated the cause could be something else, Snyder said.

The damaged Michipicoten is laid up at Keefer Terminal in Thunder Bay while investigators from the U.S. and Canada try to figure out what went wrong.

The ship began taking on water shortly before 7 a.m. Saturday about 35 miles southwest of Isle Royale in U.S. waters. The Coast Guard and the U.S. National Park Service responded, evacuating half of the Michipicoten's 22 crew members by boat. None were injured.

"They needed some crew to keep the vessel moving," Snyder said, while the rest were evacuated "out of an abundance of caution."

The ship, listing at 5 degrees, reached Thunder Bay under its own power. It was accompanied part of the way by another bulk carrier, the Edwin H. Gott, with the assistance of Coast Guard, U.S. Border Patrol and Park Service boats.

The Michipicoten was built in 1952 as the coal-fired steamer Elton Hoyt II. It was converted to diesel propulsion and rechristened the Michipicoten in 2003 when it was bought by a company that is now a subsidiary of Rand Logistics.

New Jersey-based Rand Logistics is a major Great Lakes fleet operator. Ten of Rand's vessels are U.S.-flagged; another six, including the Michipicoten, sail under the Canadian flag.