Laboratory tests show that globs of oil found on two Louisiana beaches after Hurricane Isaac came from the 2010 BP spill.
Tests run by Louisiana State University for state wildlife officials confirmed that oil found on Elmer's Island and Grand Isle matched the biological fingerprint of the hundreds of millions of gallons of oil that spewed from BP's Macondo well.
On Wednesday, BP said oil from its spill had been exposed by Isaac's waves and that the company would work to clean it up.
Ed Overton, the LSU chemist who did the state tests, said the oil found on Elmer's Island had not degraded much while oil at Grand Isle had.
Two other samples collected from another barrier island did not match the signature of oil from the BP well.