Federal regulators examining last month's oil spill off the California coast said the pipeline in question showed signs of extensive corrosion.

A pipeline system operated by Plains All American, which has headquarters in Houston, leaked as much as 2,500 barrels of oil in Santa Barbara County in mid-May. About 500 barrels may have reached the waters off the coast of Refugio State Beach in a release the Environmental Protection Agency said was the worst spill 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. PHMSA said it found "extensive" corrosion, with walls degraded by as much as 74 percent of their original thickness in some locations.

"The rupture characteristics at the failure site indicate a longitudinally oriented opening approximately 6 inches in length and located in the bottom quadrant of the pipe," PHMSA said its findings.

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

The PHMSA's report was published along with a corrective action order sent to the pipeline company. Plains under the terms of the order has 60 days to examine other pipelines in the area. Continued operation of the system without corrective measures "is or would be hazardous to life, property, or the environment," the order said.

Plains in a response update said about 290 barrels of an oily-water mixture has been recovered to date. The oil sheen has dispersed. For wildlife, 87 oiled birds and 53 mammals were recovered dead.