VALLEJO (CBS SF) — Residents in Vallejo are urged to shelter in place as crews work to determine the cause of a strong odor detected Tuesday evening.

Kaiser Hospital in Vallejo is being inundated with patients who say they are having trouble breathing.

READ MORE: COVID Reopening: SF Restaurants Get Set For Indoor Dining Under Red Tier

Fire officials said crews have not detected any abnormal readings on portable gas monitors and the U.S. Coast Guard has confirmed that there is an oil sheen on San Pablo Bay near the Carquinez Straight.

As of 11:15 p.m., the Coast Guard had not confirmed whether the sheen caused the odor in Vallejo.

Vallejo police said the first call on the odor came in at 7:20 p.m. from the western side of Mare Island.

Coast Guard officials said they received a call from a ferry captain around 8 p.m. who reported seeing the oil sheen. Coast Guard officials went to the site and confirmed the sheen. The Coast Guard’s pollution control team is now working to control the sheen.

READ MORE: SJ Volunteers Conducting Foot Patrols In Response To Rash Of Attacks On Asians

Solano County fire officials are continuing to determine the cause of the smell, which was detected largely in southeast Vallejo neighborhoods as well as in Benicia.  PG&E crews are assisting in the investigation.

Vallejo fire officials announced on Twitter that the smell “has not been determined to be natural gas, refinery burnoff, or otherwise.”

At 9:54 p.m., the City of Vallejo announced that they have partially activated their emergency operation center as well as the city’s hazmat team, but that the source of the odor remains unknown.

MORE NEWS: Federal Judge Bars Sausalito From Clearing Homeless Encampment In Dunphy Park

As of 10:45 p.m., police said there have been no gas leaks detected in the City of Vallejo.