Swimming was banned at Indiana Dunes State Park after a silvery substance was found floating on the water.

8:00 p.m. CDT, June 18, 2013

The U.S. Coast Guard and Indiana environmental officials continued Tuesday to investigate the origins of a silvery substance that was found in southern Lake Michigan, causing some beaches to clear bathers from the water.

Swimmers along Porter Beach at the Indiana Dunes National Lakeshore came out of the water Monday afternoon with an "oily substance" on their bodies and saw a silver sheen in the lake, according to the Coast Guard and the Indiana Department of Environmental Management.

Preliminary tests indicate the substance included D-gluconic acid, a mild acid used to clean metals, and tricalcium orthophosphate, an additive found in food and fertilizers, said Dan Goldblatt, a spokesman for the Department of Environmental Management.

"Initial reports make it look like it's not something that's highly toxic," Goldblatt said. "Once we know more we can make a better determination."

A current carried the sheen several miles toward Michigan City, Ind., but investigators found nothing except pollen in the water when they arrived Monday.

The Coast Guard's safety marine unit in Chicago has viewed a week's worth of video showing vessels in the Port of Indiana, but have not seen anything spilling from the ships, said Petty Officer 3rd Class Christopher Yaw.

The unit also talked to officials at a steel and mining company along the lakefront but determined it did not cause the sheen because only iron-ore based products have left the site, Yaw said.

Monday night's rains and natural breakdown dissipated the sheen from the water. Those who came in contact with the substance reported no ill health effects, Indiana environmental officials said.

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Officials identify silvery substance at Indiana beaches