An oily spill or runoff of unknown size shimmered along the Virginia side of the Potomac River on Thursday as at least six government agencies scrambled to determine its source, size and composition. (Dean Naujoks/Potomac Riverkeeper Network)

U.S. Coast Guard officials said Friday that the oil sheen spotted in portions of the Potomac River over the past two days, coating some wildlife, appears to have subsided.

Coast Guard Lt. David Ruhlig, who is leading the multi-agency investigation into the spill, said the sheen has largely dissipated and is now limited to the area around Roaches Run Waterfowl Sanctuary in Arlington.

The oil sheen was first discovered Wednesday afternoon. The Potomac Riverkeeper Network, a nonprofit organization that monitors the river, was one of the first to sound the alarm. In a statement Friday, it said that it was “extremely concerned about the spill” and called on authorities to “investigate and prosecute the party responsible for this ongoing pollution of our nation’s river.”

At least 18 geese and a duck have been discovered covered in oil at Roaches Run, just north of Reagan National Airport, and elsewhere in the Potomac River watershed.

Virginia environmental officials said Friday that they are investigating the cause of the oil sheen.

An oily spill or runoff of unknown size shimmered along the Virginia side of the Potomac River on Thursday as at least six government agencies scrambled to determine its source, size and composition. (Dean Naujoks/Potomac River Keeper)

About 13,500 gallons of mineral oil leaked from a Dominion Virginia Power company transformer station late last month, but state officials said it was too early to tell whether that was the cause of the river contamination.

William Hayden, spokesman for the Virginia Department of Environmental Quality (DEQ), said his department is working with the Coast Guard and Maryland environmental officials to determine the cause.

“We just don’t know” that the Dominion leak is the cause, Hayden said. “We’re trying to sort that out, but it’s taking a little time.”

A Dominion Power spokesman said the sheen does not appear to be connected to the earlier oil spill, most of which has been cleaned up.

“We have no evidence to indicate it’s from a recent mineral oil spill at a Dominion substation,” David Botkins, the spokesman, said in a statement. “We have conducted an exhaustive cleanup of the spill in coordination with local, state and federal officials. We will continue to work closely with the local, state and federal officials.”

A DEQ report says the storm-water systems near Roaches Run were inspected Thursday and no oil sheen was evident.

However, the report states, the department received numerous calls about oil seen in various portions of both the Potomac and Anacostia rivers.