VALLEJO (CBS SF) — Arabian crude oil being moved from a tanker to the Phillips 66 refinery in Rodeo was the source of oil sheens visible on the San Pablo Bay last month, U.S. Coast Guard officials announced Thursday.

While the Coast Guard Marine Safety Lab determined that the crude oil entered the San Pablo Bay as it was transferred from the Yamuna Spirit oil tanker to the Phillips 66 refinery on Sept. 20, the Coast Guard could not determine whether the oil originated from the Yamuna Spirit or from the Phillips 66 facility.

As a result, both the vessel and the refinery will be responsible for the cost of the response by the federal government.

The Yamuna Spirit, a 274-meter-long vessel that sails under the Bahamian flag was examined by the Coast Guard and departed the port. It is now off the southern Coast of Mexico, according to the website MarinTraffic.

The Coast Guard reported that they couldn’t conclusively determine what caused a mysterious odor experienced in Vallejo the same day the oil sheens were detected.

By Hannah Albarazi – Follow her on Twitter: @hannahalbarazi.