The U.S. Environmental Protection Agency estimated Wednesday that at least one million gallons of oil may have spilled into southern Michigan's Kalamazoo River.
Enbridge Incorporated, an energy company based in Calgary, Alberta, owns the pipeline that leaked the oil.
Gov. Jennifer Granholm criticized the Canadian company's efforts to contain the oil Wednesday night. She said it will be a "tragedy of historic proportions" if the oil reaches Lake Michigan, which is located about 80 miles downstream.
Granholm called on the federal government for more help, saying resources being marshaled by the EPA and Enbridge were "wholly inadequate."
Enbridge said earlier Wednesday that it had redoubled its efforts to clean up the mess. Chief executive Patrick D. Daniel said the company had made "significant progress," though he had no update on a possible cause, cost or timeframe for the cleanup.
A crew of about 400 people were working to clean up the spill on Wednesday. Both company and EPA officials have said oil is no longer leaking.