I ran a fuel bunkering operation in Duluth-Superior for 16 years and the captain of our lighter vessel helped put the last N6 oil on board the Fitz. He knew most of the guys who worked for Columbia Steamship Company, the ship's owner. He said the regular cook was ill and missed the last voyage. His replacement cook was a spooky guy and on the evening they left Superior he mentioned that he felt he was going to a better place sometime soon.
One cable station special I saw a few years ago came to the conclusion that the keel/hull failed completely about mid-way along it's length .
Another possibility was water got in thru open hatches, a wave hit and she dove for the bottom.
The h istory channel had a great show on it a couple of years ago.