The federal government said its preliminary report of the May 2015 oil spill at Refugio Beach in southern California found pipeline corrosion to be the culprit.

A pipeline system operated by Plains All American, which has headquarters in Houston, leaked up to 3,400 barrels of oil in Santa Barbara County in mid-May. The company in November indicated the spill volume was around 2,960 barrels and was still working to reconcile the difference. About 30 percent was recovered during remediation efforts.

Migrating to Refugio Beach near Los Angeles and into the ocean, the Environmental Protection Agency characterized the spill as the worst in California in the last 25 years.

The federal Pipeline and Hazardous Materials Safety Administration released their preliminary findings of the integrity of the pipeline system last year. PHMSA said it found "extensive" corrosion, with walls degraded by up to 74 percent of their original thickness in some locations.

"Preliminary findings indicate that the root cause of the Line 901 failure was external corrosion," the PHMSA said in its latest report.

The federal report found the pipeline ruptured at 56 percent of its maximum operations pressure. During surveys, the report said the extent of external corrosion was not accurately assessed and that corrosion control systems enacted by Plains were under-performing.

Plains said it had conducted a system inspection two weeks before the May 19 spill, though results weren't returned until after the incident.

A court ruling issued in the late 1980s allowed the pipeline's former operators to use the pipeline without an automatic shutoff valve after a county decision to require them. Plains purchased the pipeline in 1998.

Plains last year said it's "much safer" for a controller who understands the hydraulics of crude oil pipelines to shut down operations using a controlled and planned sequence of events. The sequence of events outlined in the latest report from the PHMSA found oil was leaking from the line for about two and half hours before it was confirmed by field personnel.

Plains had no statement on the latest PHSMA report.