SEATTLE -- An oil sheen was spotted early Friday on Seattle's Elliott Bay, prompting inspection by the Coast Guard and state Ecology Department.

A few hours after the discovery, investigators determined it was a "petroleum product," Coast Guard Petty Officer Jeff Pollinger said.

"We suspect it's a mix of motor oil and diesel although we won't know for sure until testing is complete," he said.

Chopper 7 pilot Clark Stahl spotted the sheen, on the north side of the bay, as he was on approach to Boeing Field just before 7:30 a.m. Friday.

Pollinger said it was spotted by a tug boat and reported to the Coast Guard at 7:10 a.m.

It was also was reported by four planes that flew over the bay on a landing approach to Boeing Field.

Described as a "light, patchy sheen," it was estimated to be less than a quarter square mile in size, Pollinger said. Crews were placing absorbent pads in the water to soak it up.

The sheen spread between Smith Cove and Pier 70, said Ecology Department spokesman Larry Altose.

"Hopefully we got it soon enough to get it stopped," Altose said.

The Coast Guard sent a 41-foot boat and a helicopter to try to identify the cause, and others patrolled the shore on foot.

Ecology also chartered a helicopter to fly over the area so investigators could get a better look at the sheen, Altose said.

Video from Chopper 7 showed the sheen over a wide area of northern Elliott Bay. Two large ships were in the area.

The source was not immediately known.

There was no sign of damage to wildlife from the sheen, Pollinger said, nor did it appear to have reached the shore.