Plains All American defended pipeline infrastructure in California after last week's spill, saying automatic shutoff valves were not the best safety option.

Plains is working with state, local and federal regulators to remove part of its Line 901 from a region near Refugio State Beach in Santa Barbara.

A unified command center established for the spill said the worst-case estimate is that 2,500 barrels of oil was released from the pipeline. About 500 barrels may have migrated from the pipeline into the waters off the Santa Barbara coast.

On-scene investigators, including a metallurgist, are working to determine the cause of the breach. Plains, which has a checkered record in terms of pipeline operations, said it manually shut down Line 901 after detecting a problem.

"We do not feel it is prudent to install an automatic shutoff valve for safety reasons," the company said in a statement. While such valves are suitable for some types of liquids, the pressure buildup in a high-volume pipeline like Line 901 would be like "slamming a car into park when driving down the freeway."

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

Plains 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.

California Gov. Jerry Brown declared a state of emergency for Santa Barbara County early last week.

Plains said nearly all of the visible sheen off the coast of Santa Barbara has been removed.