The Orange County District Attorney's Office confirmed that a criminal investigation has been opened into the incident. Federal and state investigations are also underway involving the U.S. Coast Guard, National Transportation Safety Board and California Department of Fish and Wildlife, among others.
One possible theory is that a ship's anchor was dragged along the ocean floor, rupturing and displacing the pipeline.
The Coast Guard has been looking at a German cargo ship, the Rotterdam Express, which was tracked making several unusual movements in the area of the pipeline at the time of the rupture. But the ship owner has said the third-party tracking data was incorrect and the vessel did not move as indicated after anchoring.
Coast Guard personnel boarded the Rotterdam Express in Oakland on Wednesday to further the investigation. The federal agency did not disclose the results of that search.
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A spokesman for the shipping company told Eyewitness News that the Coast Guard "investigation is closed" regarding the Rotterdam Express, and it was allowed to leave Oakland. The ship is now heading to Mexico.
But the Coast Guard has not confirmed that its investigation has cleared the ship of involvement in the rupture.
Satellite images provided to Eyewitness News indicate at least seven ships passed over the pipeline around the estimated window of time the leak began.
That includes a large ship, the Zim Sao Paolo, that was seen crossing over the pipeline twice, although it apparently did not anchor.
"We're not saying they caused damage to the pipeline but certainly they were in the area during the day on Friday when in our best guess that pipeline was damaged," said John Amos with Skytruth, a nonprofit group that uses satellite images to monitor environmental threats.
Officials on Thursday also updated their estimates of the size of the spill.
Initial estimates in the days after the spill were that at least 126,000 gallons of oil had leaked into the ocean. Officials are now saying a lower number is within the range of possibilities but the analysis continues.
Authorities say their current estimates range from a minimum of about 25,000 gallons to a worst-case scenario of more than 131,000 gallons.
Wildlife activists say they have recovered at least 19 live birds and five dead birds affected by oil and continue to investigate reports of more injured and oiled wildlife.