The U.S. Coast Guard is investigating reports of an oil sheen on waters near a northern California community where local authorities are trying to determine the source of a strong odor.

The Coast Guard didn't spot a sheen around Vallejo from the air on Tuesday but said crews will look again Wednesday morning.

Fire officials, meanwhile, are telling people in the city to shelter in place.

The Fire Department says gas monitors are registering safe levels, but it has asked residents to close windows and turn off air conditioning units. Fire officials say the odor is not linked to any natural gas or refinery burn off from nearby facilities.

Vallejo is on the San Pablo Bay, north of San Francisco.