South Floridians enjoying a pleasant Saturday at the beach found themselves stepping in dark, greasy balls of oil, possibly due to a spill offshore, officials said Saturday. The source of the tar remained unknown Sunday morning as the Coast Guard investigation resumed.

The U.S. Coast Guard has been searching for tar balls that began showing up on Florida beaches over the weekend. So far, the source of the black muck remains a mystery.

Fort Lauderdale beach reopened to swimmers Sunday after an oily substance washed ashore. The U.S. Coast Guard is still working to determine the source of the substance but it’s suspected that it could be from an oil spill.

After aerial flights and physical searches, the Coast Guard says the little blobs of oil are no longer floating ashore.

Mysterious tar balls washed up on several Florida beaches, forcing closures and triggering an investigation by the U.S. Coast Guard. Fort Lauderdale’s shoreline was among the most impacted, after the strange oil slicks shut down swimming.

Several beaches across Broward County were closed on Saturday after reports of tar bar washed on shore, officials said.

Fort Lauderdale Fire Rescue announced that the oily substance on the beach prompted lifeguards to tell beachgoers to stay out of the water from Riomar Street south to the Port Everglades inlet. That makes up about half of Fort Lauderdale’s beaches, said Guzman.