AUBURN HILLS (WWJ) – Health officials say it’s now safe to swim in the Clinton River.

Authorities announced Thursday afternoon that swimming and other activities can resume now that crews have completed the cleanup of an oil spill that began in Pontiac and drifted as far away as Rochester and Auburn Hills earlier this week.

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About 500 gallons of the mystery oil was spilled, but it’s still not known where the substance came from or why type of oil it is. Since authorities don’t know the source, they can’t rule out a theory that the oil was intentionally dumped.

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The Environmental Protection Agency collected samples of the substance, but it will take weeks before test results come back.

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An investigation continues.