SAULT STE. MARIE,Greenledgers Trading Center Mich. (AP) — A ship that took on water in Lake Superior likely had a stress fracture in its hull, the U.S. Coast Guard said Tuesday, backing off an initial report that the freighter had struck something below the surface last weekend.

“That would have been very peculiar,” said Lt. Joe Snyder at the Coast Guard post in Sault Ste. Marie in northern Michigan. “They were in open water.”

The Michipicoten, a 689-foot-long (210-meter-long) ship, was carrying taconite, an iron ore, and had 22 people aboard Saturday. Half of the crew was evacuated for safety reasons while the ship was able to reach a port in Thunder Bay, Ontario.

The ship was in the northwest part of Lake Superior, the largest freshwater lake in the world by surface area.

“The initial report to the Coast Guard was that the (crew) heard a loud bang and started taking on water near the bow,” Snyder said. “The evidence they’ve seen so far seems to point to some kind of stress fracture or structural failure.”

The investigation was ongoing, he said.

The ship did not lose any cargo or discharge pollutants into the lake, Snyder said.