BP says it is assessing how much crude oil entered Lake Michigan following a malfunction at its northwestern Indiana refinery.
BP spokesman Scott Dean says crews have placed booms across a cove at the company's Whiting refinery where workers discovered the oil spill Monday afternoon.
Dean says BP believes the oil released during an oil refining malfunction has been confined to that cove.
He says the oil entered the refinery's cooling water system, which discharges into the lake about 32km southeast of downtown Chicago.
Indiana Department of Environmental Management spokesman Dan Goldblatt says an agency staffer reported seeing a large sheen on the lake about 2am on Tuesday.
Dean says that sheen was in the cove and was no longer visible several hours later.