Skagway Mayor Mark Schaefer says they may have found a clue to what caused the Alaska town's state ferry dock to sink last week.

The exact cause of the sinking won't be known until the dock can be refloated and inspected, but Schaefer says meters on the town's water supply show an unusual amount of water use in the hours before the dock sank.  

Schaefer says the community uses an average of 300,000 gallons of water in a 24-hour period. But before the ferry dock sank, Schaefer says the city registered 800,000 gallons of water use.

He says a line under the dock that is used to re-supply the ferries with water may have ruptured.

"So essentially we think we probably flooded the float and sank it that way, but we're not sure yet," he said.

Schaefer says efforts continue to re-float the ferry dock. Two barge crews are pumping water from two dozen concrete flotation tanks on the dock.

The dock sank early last Thursday morning and now sits on the ocean floor.

Alaska Marine Highway System ferry service to Skagway has been cancelled until at least May 9.