An unknown chemical leaked into the Ohio River last night, killing several fish. But water samples taken since then show no sign of any contamination.A sheen was noticed on the river’s surface by cameras at Dow Chemical’s plant, south of Rubbertown.The Lake Dreamland Fire Department first responded to the call and alerted the Coast Guard and Health Department.Chief John Wilkinson says no one is sure where the chemical came from or what is was, despite water sampling.“MSD ran a test, could find nothing," he said. "The Health Department ran a test, could find nothing. Dow Chemical took the samples in. I was notified this morning that they could find nothing in it either.”Wilkinson says there are four plants in his district upriver from the place where dead fish were found, but the Coast Guard followed the sheen past that. Now, the Kentucky Division of Water is investigating the leak.Updated here.