OAKLAND  (AP) — An unknown, viscous substance has been found in dozens of seabirds, including 20 dead ones, on the San Francisco Bay’s eastern shores, rescuers and state wildfire officials said Saturday.

The mysterious gunk came to officials’ attention late Friday when some birds were seen looking distressed in the water and stranded on land in Alameda, the Hayward shoreline and the San Leandro marina, said Barbara Callahan, interim director of International Bird Rescue.

In all, 70 seabirds were taken to the rescue center and 20 dead ones were recovered in the field.

Preliminary laboratory tests found the thick substance, which is clear to pale gray in color, is not petroleum or toxic, said California Department of Fish and Wildfire spokesman Andrew Hughan. It doesn’t smell, isn’t fish or vegetable oil-based, and can’t be removed by soap.

“We don’t know what it is,” Hughan said.

The substance damages the birds’ feathers and ability to remain waterproof, which can lead to hypothermia in the bay’s cold water, Callahan said.

The Coast Guard flew over the area and didn’t see a spill or other indication of where the substance came from.

Further lab tests of the substance will begin Tuesday after the holiday weekend, and a necropsy will be performed on the dead birds, Hughan said.