WASHINGTON (AP) — An environmental group is offering $1,000 to anyone with information about what caused an oily sheen on the Potomac River.

The Washington Post reports (http://wapo.st/2guedFf ) that the Potomac Riverkeeper Network is seeking tips about whoever spilled the mysterious substance. The group will send the information to the Environmental Protection Agency, which is leading the investigation.

Local officials say the substance contains particles of light oil, like mineral oil or hydraulic fluid.

The sheen was spotted this week about 45 miles upstream from the District of Columbia near Point of Rocks, Maryland. It was last seen near Sterling, Virginia.

Investigators are interviewing businesses and facility owners along the Potomac and the Monocacy River.

Dean Naujoks of the Potomac Riverkeeper Network says the group is treating it "as an environmental crime."

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Information from: The Washington Post, http://www.washingtonpost.com