While less than half of the California beaches soiled by last month's spill are cleaned, those remaining have only trace amounts of oil, a unified command said.

About 500 barrels of oil may have reached the waters off the coast of California when Line 901, a pipeline system operated by Plains All American, leaked May 19. All told, about 2,500 barrels of oil spilled in a release the Environmental Protection Agency said was the worst spill in California in the last 25 years.

A unified command, led by Plains, local, state and federal officials, said about 44 percent of the 96.5 miles of shoreline soiled by the spill have met their cleanup goals. Most areas show only trace amounts of oil.

"Clean areas will continue to be monitored for re-oiling which may occur, especially with the natural oil seeps in the area," the latest update read.

The federal Pipeline and Hazardous Materials Safety Administration released their preliminary findings of the integrity of the pipeline system last week, saying it found "extensive" corrosion, with walls degraded by as much as 74 percent of their original thickness in some locations, on sections of Line 901.

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

Sheen has long since dissipated from California waters, though response crews continue to remove tar balls, soiled sand and vegetation from beaches impacted by the spill. So far, 136 birds and 67 mammals have been found dead since the spill.

"No additional oiled live wildlife has been found in the response area since June 5," responders said. "Not all of the dead animals collected have been oiled. Oiling itself is not an indication of the cause of death."