Days before trial, charges have been dropped against two people who allegedly discharged oily water into Lake Huron from a tugboat in 2014.
Oil was spotted by air east of Cheboygan and east of Alpena.
One oil sheen was 600 feet long.
Engineer Jeffrey Patrick and assistant engineer William Harrigan were charged with conspiracy and one other crime while aboard a boat named Victory.
Prosecutors requested a federal judge in Bay City to dismiss the indictment last week.
Court documents show no explanation.
Messages seeking comment were left for defense lawyers and prosecutors on Monday.
The trial was set to start on Tuesday.
Last year the U.S. Environmental Protection Agency said that this case would send a “strong message” about enforcing laws.