At 7:55 p.m. firefighters reported on social media that the strong odor of natural gas had been reported in the Glen Cove and Beverly Hills Park neighborhoods.
"If you smell gas outside shelter in your home, close windows, turn off air units," firefighters said on Twitter.
By 8:02 p.m., the fire department had received more than 500 reports regarding the odor.
Crews are using portable gas monitors, but have not picked up any abnormal readings, firefighters said. They have, however, determined that the odor is not natural gas or burn-off from a refinery.
As of 9:18 p.m. the investigation was ongoing and the shelter in place order was still in effect and PG&E crews were assisting in the investigation.
Residents have been advised to close their windows and turn off any air-conditioning systems, according to Vallejo city spokeswoman Joanna Altman, who said that more than 800 calls had been received by dispatch in a video posted to social media around 10 p.m.
An unknown number of residents have reportedly gone to a hospital, Altman said.
Coast Guard Lt. Steven Dross said that a sheen has been confirmed on the water in a number of mostly-connected locations in and around Vallejo as well as San Pablo Bay and the Carquinez Straits.
Coast Guard personnel at Station Vallejo can see the sheen on the water from the docks, according to Dross, but the nature of the substance on the water has not yet been determined and it is not yet clear whether the sheen is connected to reports of an unusual odor nearby.
Pollution responders are currently investigating, Dross said.
ALERT: strong order of natural gas reported in Glen Cove and Beverly Hills Park neighborhoods. VFD and PG&E investigating.— Vallejo Firefighters (@VallejoFire) September 21, 2016