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MIAMI, May 19 (Reuters) - Tar balls found on beaches in the Florida Keys this week are not from the Gulf of Mexico oil spill leaking from a well owned by BP BP.L, the U.S. Coast Guard said on Wednesday, citing laboratory tests.

The news came as a temporary relief to Florida’s tourism authorities, who are already reporting negative market impact from the month-long spillage from BP’s leaking undersea well, the source of a huge slick that has already dumped oil debris ashore on the coasts of Louisiana, Mississippi and Alabama.

The source of the tar balls has not been determined, the Coast Guard said in a statement.

Coast Guard personnel and pollution experts had found around 50 tar balls in recent days in Florida’s Lower Keys, a mecca for divers, snorkelers, fishermen and beach goers. They had sent them to a specialist laboratory to test whether or not they came from the Gulf of Mexico spill.

“The results of those tests conclusively show that the tar balls collected from Florida Keys beaches do not match the type of oil from the Deepwater Horizon oil spill in the Gulf of Mexico. The source of the tar balls remains unknown at this time,” the Coast Guard said in a statement.

But it remained on the alert for oil contamination.

“The conclusion that these tar balls are not from the Deepwater Horizon oil spill incident in no way diminishes the need to continue to aggressively identify and clean up tar ball-contaminated areas in the Florida Keys,” said Captain Pat DeQuattro, commanding officer of Sector Key West.

Despite the laboratory result, Florida Keys authorities are still preparing for possible impact from the Gulf spill as many forecasters see some oil from it being sucked by a powerful ocean flow, the Loop Current, around the Florida Keys and perhaps even up to Miami beaches. (Reporting by Pascal Fletcher and Jane Sutton; Editing by Doina Chiacu)