GRASSY KEY, Fla. (AP) - The Coast Guard is investigating after tar balls were found off the Florida Keys, but the agency hasn't determined whether they came from the massive Gulf of Mexico oil spill.

Coast Guard officials flew over the area and collected samples after an oily substance and tar balls were reported Thursday a few miles off Long Key. That's about the midpoint of the 110-mile island chain.

Hundreds of tar balls wash up annually on Florida beaches. Dozens of samples have been sent for testing, but Florida officials reiterated Thursday that none have been tied to the Louisiana spill. This marks the second "batch" of tar balls to wash ashore in the Keys since the Gulf oil spill. The first batch of testing revealed no link to the BP oil spill in the Gulf.

Scientists say oil has drifted into a major ocean current that could carry it through the Keys and up the East Coast.

Authorities are working to protect dolphins at a nearby research center.