Workers with Miller Environmental Group collect absorbent material used to soak up oil from a spill on the surface of the water near Reagan National Airport on Monday in Arlington. (Kate Patterson/For The Washington Post)

Dominion Virginia Power said late Friday that the oil that fouled a Northern Virginia waterfowl sanctuary and the Potomac River early this month came from its Crystal City substation.

Five hours after Coast Guard Cmdr. Michael Keane said that all the oil his interagency task force sampled came from a common source, Dominion said in a statement that a previously reported 13,500-gallon spill of mineral oil at its Crystal City substation was the likely cause.

“Now that we’ve had the opportunity to look at the data the Coast Guard collected, we concur with their findings and accept responsibility for the spill,” Dominion spokesman Chuck Penn said in an interview.

“As we have stated from the very beginning when we were made aware of the oil sheen, we would have no hesitancy in accepting responsibility should testing link our January 24 transformer spill to the sheen,” the company’s statement added. “We will move with all due haste to work with the agencies to ensure the remaining cleanup work is done.”

Dominion identified the spilled oil as mineral oil, used at a substation in a transformer.

The admission clarified what had been a less-than-crystal-clear conclusion from the Coast Guard, which had said that although all the oil came from a common source, no culprit would be identified. The oil collected from Roaches Run Waterfowl Sanctuary and the Potomac matched oil found at the Dominion substation, at 18th and South Fern streets, as well as at an adjacent storm-water drain, but Keane would not say whether Dominion was the source of the spill.

As late as 3 p.m. Friday, Dominion insisted that it had cleaned up 95 percent of the January spill, and the rest was believed to be contained within machinery at the site. But by 6 p.m., the utility had reviewed the data and accepted responsibility.

Pam Faggert, Dominion’s chief environmental officer, had said that the utility collected its own oil samples and sent them to an independent laboratory for an assessment separate from the Coast Guard’s.

“If it is determined that the oil in the Potomac River is from the Dominion facility, we will take responsibility,” she said midafternoon.

Twenty-one birds, mostly Canada geese, died after the medium-weight petroleum coated their feathers, and 32 more are being treated by a wildlife rescue team. There were no fresh sightings of oil or birds covered in oil in the past day, Keane said, but there are remnants of the oil that coated the lagoon’s waters and shoreline.

The Coast Guard previously identified the oil in the water as medium-weight oil, similar to home heating oil but not motor oil. That weight spectrum includes mineral oil.

Petty Officer David Marin, a spokesman for the Coast Guard, said Friday evening that he expects that Dominion will probably have to pay for the investigation and cleanup, and that it may be fined.

“The Coast Guard is pleased Dominion is claiming responsibility and will sit at the table,” Martin said. “We’ll do everything we can to resolve the issue.”

Keane earlier had no estimate for how much money the investigation has cost, but he said the Coast Guard has not exhausted the $250,000 it has drawn from the federal oil trust fund.

The oil sheen stretched from the waterfowl sanctuary, just north of Reagan National Airport, to Dyke Marsh, just south of the Capital Beltway’s Woodrow Wilson Bridge.

Storm water, draining from the highly urbanized Crystal City and Pentagon City areas of Arlington County, dumps into Roaches Run, which feeds into the Potomac.