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RICHMOND,CA (AP) - Federal investigators say a corroded pipe that failed and triggered a fire at one of the nation's largest refineries last month had walls as thin as a penny in some areas.

U.S. Chemical Safety Board officials said Tuesday that a key part of their probe into the fire at the plant in Richmond, Calif., is why Chevron didn't replace the pipe during a routine inspection a year ago.

The Aug. 6 blaze in the San Francisco Bay area knocked an important refinery unit offline, reducing the facility's production. Smoke from the fire sent thousands of residents nearby to hospitals with health complaints.

The Contra Costa Times reports that parts of the failed pipe had thinned to 1/16 of an inch from its original thickness of 5/16 of an inch.