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.

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.