The oily water then was discharged into the cove leading to the lake, he said. The discharge did not last long.

Beslow said the refinery was no longer discharging oil when he arrived about 9 p.m. Monday. BP officials informed him crude oil, which had not yet gone through the first stage of processing, had gotten into the No. 6 separator, which uses water from Lake Michigan to cool equipment. That water is supposed to go from a holding pond back into the lake, and was contaminated with crude oil before it was discharged through an outflow pipe, Beslow said. The equipment has a detection mechanism that immediately alerted refinery workers to the discharge.

BP said in a statement that there had been no further discharge of oil.

"Based on preliminary information, BP believes an upset at a crude distillation unit may have sent crude oil into the refinery's cooling water outfall and then into the lake. BP's investigation of the incident continues and the refinery has taken steps to prevent another discharge," the company said in a statement. "Meanwhile, response efforts continue. Lines of boom have been deployed to contain the oil and wind has blown oil toward the shore, where crews are vacuuming it out of the water and cleaning the limited quantities that have reached land between the refinery's wastewater treatment plant and a nearby steel mill."