A large section of the pipeline off Huntington Beach, south of Los Angeles, was found to have been hit and dragged along the seabed from its original location.
Officials say they do not know when the 13-inch crack in the pipe began leaking oil and they have not yet identified any particular cargo ship as the main suspect.
“We’re going to be looking at every vessel movement over that pipeline, and every close encroachment from the anchorages for the entire course of the year,” said Coast Guard Captain Jason Neubauer, chief of the office of investigations and analysis.
The pipeline is situated outside the Long Beach-Los Angeles port, which is the biggest in the country and handles 4,000 cargo ships a year from around the world.
Captain Neubauer said investigators were narrowing their search to ships large enough to move a 4,000ft section of pipeline.
Officials believe that the initial anchor strike took place after a survey of the pipeline a year ago when it was still in its original location.
The crack in the pipeline , which was built in 1980, suggests that it had withstood an initial impact but had become weaker over time and more likely to fail, said Ramanan Krishnamoorti, a petroleum engineering professor at the University of Houston.
The Coast Guars estimates that between 25,000 gallons and 132,000 gallons of oil spilled in the incident, which has seen 10 birds die as a result, and 25 more treated.
Although some of the beaches in Orange County have now reopened the public is still not allowed back in the ocean.
At least 17 incidents of oil pipelines being hit by anchor strikes have taken place since 1986, according to the Associated Press.
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