An oil sheen spotted off the Southern California coast last week was possibly caused by a natural seep from the ocean floor, but the exact source still isn't known, U.S. Coast Guard officials said Monday.

Authorities detected the 2.5-mile-long (4-kilometer-long) oil sheen Friday morning off Huntington Beach. Crews recovered roughly 85 gallons (322 liters) of oil from the water and 1,050 pounds (476 kilos) of oily waste and tar balls from the shoreline, the Coast Guard said in a statement.

COAST GUARD ESTIMATES MORE THAN 1 MILLION GALLONS OF CRUDE OIL SPILLED OFF COAST OF LOUISIANA

Lab tests so far have failed to definitively identify the oil source, but preliminary analyses determined it was not a refined product like gasoline or diesel, officials said.

"The lab results were also inconsistent with archived samples from oil platforms in the area," the statement said. The samples are more characteristic of "freshly produced" oil typical of natural seepage, according to officials.

Authorities said initially that there were no reports of spills or leaks from oil platforms operating offshore and the sheen did not appear to be growing, which led them to suspect it was a one-time discharge of oil from the ocean floor, which are fairly common.

By the time responders conducted flights over the area Saturday, they could no longer see any recoverable sheen. There were some clumps of tar along the beach but not enough to warrant a weekend closure, and there did not appear to be any public health threat, officials said.

The Coast Guard said three live birds — a cormorant, a loon and a grebe — were found sullied with oil and were being treated.

A 2021 leak in an offshore pipeline sent blobs of crude oil washing ashore in Huntington Beach, a surf-friendly city about 30 miles (48 kilometers) southeast of Los Angeles.