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

The oil that leaked into the waters of a waterfowl sanctuary and the Potomac River last week was fuel oil, similar to home heating oil, officials investigating the spill said Monday, but they do not yet know its source.

The incident commander of a multi-agency task force investigating the spill said he has ruled out the theory that the oil came from runoff after the Jan. 22-23 blizzard. But he has not eliminated the possibility that it was left over from a Jan. 24 mineral oil spill at a nearby Dominion Virginia Power substation.

Other than runoff from the storm, “right now I have not ruled out any potential sources,” Coast Guard Cmdr. Michael Keane said at a news conference at Gravelly Point, just north of Reagan National Airport and across George Washington Memorial Parkway from the Roaches Run Waterfowl Sanctuary, where most of the oil collected. Laboratory tests are underway to see if the oil recovered from the scene matches the oil at the Dominion site.

Dominion is cooperating with the investigation, but spokesman Charles Penn said that 90 percent of the 13,500 gallons of oil spilled last month has been cleaned up and the rest is probably embedded in soil that has been removed.

The oil sheen was first spotted Wednesday and officials thought they had it contained by Friday. But on Sunday, officials said another “outfall” had been spotted in the waterfowl sanctuary, which is also a tidal lagoon.

Coast Guard personnel and Arlington County storm water specialist Diane Handy, center, read a dipstick indicator during their investigation into the source of an oil spill. (Kate Patterson/For The Washington Post)

Some oil could still be seen Monday in the lagoon and along the shoreline outside of the containment booms toward the Potomac. But Keane said the booms, which are replaced daily, are doing their job and the oil sheen that is still evident in the area is the result of the tides pushing some of what was captured back into the lagoon and along the shoreline.

The area around Roaches Run is a spaghetti bowl of highways, railroad tracks and a former industrial site that was contaminated with PCBs and other toxins before Arlington County cleaned it up and built a recreational park above it five years ago. The Pentagon is within sight, high rises have gone up and the jets landing at the airport streak overhead constantly. That doesn’t take into consideration what’s underground.

[Environmental groups, residents express concern about oil in Potomac]

One of the unknowns of the incident is the size of the oil leak, which at one point stretched as far south as Dyke Marsh, just beyond the Woodrow Wilson Bridge.

“It’s extremely difficult to judge how much oil is in there,” Keane said. “One tablespoon of oil can cover an entire football field. In our opinion, this is not a significant [sized] spill. We regard it as relatively minor.”

Keane added later that because of the location and the impact on wildlife, the Coast Guard, the Virginia Department of Environmental Quality, the D.C. Department of Energy and Environment, the National Park Service, the U.S. Department of Fish and Wildlife and Arlington County brought multiple resources to the investigation, including helicopter flights and boat crews that examined the waterways.

About 30 birds had their feathers oiled from the material, and a Canada goose died after it was captured. The others, all geese except for a single duck, are being rehabilitated at Tri-State Bird Rescue & Research in Newark, Del.

Coast Guard Cmdr. Michael Keane holds a news conference about the oil spill near Ronald Reagan National Airport on Monday. (Kate Patterson/For The Washington Post)

A Coast Guard strike team skilled in descending into tight spaces was one of those resources, and a team accompanied Virginia and Arlington officials as they peered down storm-drain manholes in the Pentagon City area, finding traces of oil only at the bottom of one hole behind an apartment building on South Eads Street.

Using sewer and storm-drain maps, the team cleared a path through bushes to find a manhole in a dog run, maneuvered through an active construction site to find another and located another on a busy street corner. Except for that one trace, no other oil was spotted Monday.