Initial tests show degraded fuel oil is what spilled into the Potomac River near Reagan National Airport last week.
Authorities are still waiting for test results on additional samples, U.S. Coast Guard Cmdr. Michael Keane said Monday at a news conference. The source of the spill has not been found.
“There is a chance that we might not find out who the actual responsible party is,” Keane said.
Authorities checked manholes Monday to try to eliminate possible sources of the substance.
Also Monday, additional floatation devices were deployed to help contain the spill, which stretched eight miles along the Potomac River from just about National Airport down to the Wilson Bridge.
The oil sheens have mostly dissipated but are still visible, Keane said. Tidal action could create more rainbow sheens.
The oil was spotted in the water Wednesday and believed to be caused by snow runoff, Keane said, but when it continued Friday, unified command was set up.
Keane said the spill was "fairly minor" and the Coast Guard is working to minimize the environmental impact.
The number of oiled birds being cared for at an animal-rescue facility now stands at 30 -- 29 Canada geese and one mallard duck -- but one wild goose has died.