IMAGES FROM CHOPPER 7Slideshow: Spill Stretches From Vashon Island To Commencement Bay

Fog grounded helicopters Friday morning, so officials couldn't immediately get a look at how far the oil spread, said State Ecology Department spokesman Larry Altose.

Video: Sheen Spreads With Tide

He said the Fish and Wildlife Department was looking for damaged wildlife. Two oil-covered birds were seen.

The source of the spill remained undetermined early Friday. The Coast Guard said it appeared to be a heavy-grade industrial oil that can coat beaches, form tar balls and does not easily evaporate.

Cleanup operations began around midday Thursday and 10 skimmers were deployed by several contractors by the end of the day, Altose said.

The Coast Guard was reviewing a list of ships that had been in the area, trying to narrow the possible sources of the spill, Petty Officer Adam Eggers said. The list was not released.

The spill, though relatively small, was one of the few of significance to occur so far south in Washington's inland marine waters, and its impact was magnified because it has not been reported by whoever was responsible, Altose said Friday.

Because of strong currents in the area, "it was already on the run when we spotted it," he said.

A spill last December at a fuel terminal near Edmonds, north of Seattle, amounted to about 4,800 gallons.

The initial report, received by telephone about 1 a.m. Thursday, indicated the spill was roughly an acre in size. By late Thursday, Ecology officials said it had spread 5 to 6 miles, forming a U shape around the southern end of Vashon Island, creeping into Quartermaster Harbor between Vashon and Maury islands and contaminating beaches on the southern parts of both islands southwest of Seattle.

Containment boom was placed in the Quartermaster and Gig Harbor areas, Altose said.

Bill Sibbitt, 51, of Indianola, a tugboat skipper, told the Seattle Post-Intelligencer he reported the spill to the Coast Guard after encountering "a solid pool of black oil" north of Commencement Bay.

"I just smelled this huge smell of oil and I thought, `Oh, God," Sibbitt said.

Even after dawn Thursday, it took several hours for thick morning fog to clear enough for spill responders to get out on the water and into the air to investigate.

"There's really very little that can be done in the dark," Altose said.

Kathy Fletcher, executive director of the environmental group People for Puget Sound, called the time lag a big disappointment.

"We know quite a lot about currents and tides in Puget Sound, and so even though it's dark ... knowing the approximate location of a spill could alert the agencies to the possibility of oil coming to shore in certain areas," Fletcher said.