FOSTER CITY (CBS SF) — A mystery gunk coating Bay Area birds and killing them started in the East Bay, but the problem may now be spreading to new places around the Bay Area.

Wildlife crews have already been devoting their energy to East Bay shores. That’s where over 400 birds have been found mucked up over the past several days. On Tuesday, they got surprising news that three birds were found dead in Foster City.

READ MORE: Health Experts Warn COVID Rapid Tests Are Less Reliable Than PCR

But rescuers didn’t find this odd viscous stuff in the water — only on the birds. It doesn’t seem to have spread from the East Bay shoreline where all the other birds have been found all gummed up from Fremont to Alameda.

The gray, sticky material has no odor and is not believed to be toxic. It coats birds’ feathers and prevents them from insulating their bodies. So far 100 birds have died.

“They’ll get some fluid, or heat, and then go up to the hospital. It’s like a MASH site here,” Director of Wildlife Emergency Rebecca Dmytryk said.

READ MORE: Splash Brothers Shoot Warriors Past Timberwolves 124-115

The rescued ones are the lucky ducks. They have been found and are being taken care of. The care and feeding is only one aspect of this operation. The other is a deep and dark mystery.

“This case gets more complicated as we go along,” said Andrew Hughan with the Department of Fish and Wildlife. “So our first priority is to find out what it is. Second priority is where it came from. And then third, is someone responsible?”

It’s unclear whether the substance is coming from a ship, being leaked from land or being done deliberately. That’s what investigators are looking into while rescuers keep going about the business of rescuing them.

MORE NEWS: Woman Found Dead During Visit With Friends at Home in Rohnert Park

To get involved in the clean up, apply with Wildlife Emergency Services.