External corrosion on a pipeline pumping crude was the root cause of the rupture in May that spilled more than 140,000 gallons of oil on the Santa Barbara coast, federal regulators reported Wednesday.
The spill occurred after pumps on the Plains All American Pipeline were shut down and restarted, sending a larger volume of oil surging through the 2-foot pipe at higher pressure, the Pipeline and Hazardous Materials Safety Administration said in preliminary findings.
Reports of corrosion on the underground pipe and mention of the pump shutdown were not new, but regulators hadn't previously announced the cause or the precise moment of the spill.
The agency said it would provide more details when it issues a final report this spring.
The spill two weeks before the popular Memorial Day weekend forced the state to close popular beaches as the oil fouled a pristine stretch of coastline and an oil sheen spread over miles of the Pacific Ocean.
More than 300 dead animals, including pelicans and sea lions, were recovered after the spill that sent tar balls drifting more than 100 miles away to Los Angeles beaches.
The spill remains under investigation by local, state and federal prosecutors. Federal regulators could issue fines against the Texas-based company.
The spill originally was estimated to be up to 100,000 gallons, but it was revised in the latest finding to 142,800 gallons.