VALLEJO, Calif. — Authorities on Wednesday were investigating whether a mysterious odor is linked to a 40-foot wide oil sheen on San Pablo Bay, north of San Francisco, the U.S. Coast Guard said.

The probe came after Vallejo residents were told Tuesday to stay inside when the odor prompted several people to go to a hospital with respiratory issues.

The shelter-in-place order was later lifted but the source of the smell had not been pinned down.

Vallejo police said the first call about the odor came from the western side of Mare Island, KNTV reported (http://cbsloc.al/2d1NMnz ).

Some ferries were cancelled Wednesday, but the service was later restored.