"Based on the analysis and where the sheen was located, the oil sheen originated from the ship while it was unloading to Phillips 66," Sawyer said.
Sawyer said refinery personnel told him the tanker, the Yamuna Spirit, was delivering Arabian Crude shortly before the oil sheens were found.
There were initial indications that the tanker and the refinery could have been tied to the spill. Phillips 66 shut down its marine terminal shortly after the first of two oil sheens were discovered. But Coast Guard and OSPR officials have repeatedly emphasized that their investigation has not been completed.
In fact, on Tuesday OSPR's Norris confirmed that investigators still do not know how much crude was spilled in the water.
A week after the probe began, the tanker left the region and Phillips 66 reopened its marine terminal.
Phillips 66 declined to comment on the investigation and activity of the Yamuna Spirit at its marine terminal.
"Phillips 66 generally does not comment on activity as it relates to our crude supply and transportation arrangements," said Aimee Lohr, a refinery spokeswoman.
Officials with Teekay Shipping, which owns the vessel, have not returned requests for comment.
The Yamuna Spirit was reportedly involved in a major spill in Nigeria in February.
Weeks before it arrived in California, the oil tanker was in the Middle East.
The vessel offloaded a portion of its cargo in Southern California days before arriving in Rodeo, according to Gordon Schremp, a state energy commission specialist.
But it's unclear how the oil might have leaked from the vessel.
"I know that Phillips did check their piping and there were no leaks," Sawyer said. "There may have been a portion of the piping (that was) not tested."
When the investigation is concluded, local environmentalists say whoever is responsible should be held accountable.
"The perpetrators need to face stiff penalties for this absolutely unacceptable oil spill," said Patrick Sullivan, an Oakland-based spokesman for the Center for Biological Diversity.
"But even the steepest fines won't undo the damage this oil has done to the bay," Sullivan said. "That's why we've got to move away from shipping dirty crude through California's fragile coastal ecosystems."
It's common for Arabian Crude to make those trips. A significant amount of oil from the Middle East is refined at facilities in the Bay Area and the rest of California.