WASHINGTON (AP) - District of Columbia officials are hoping to learn exactly what the substance was that sparked fears of an oil spill on the Anacostia River.

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A substance resembling oil was initially spotted on Monday, and a rudimentary test indicated the presence of a petroleum-based substance in the water. But when the Coast Guard followed up on Tuesday, it found no oil. Nothing stuck to absorbent booms either, and there was no apparent harm to vegetation or wildlife.

Christophe Tulou, director of the district Department of the Environment, has said the substance is now believed to be dust or silt that was disturbed during strong rainstorms Sunday and Monday.

The substance is being tested at two labs, and a department spokeswoman says results are expected back on Thursday.