Puerto Rico ? Miles of rocky shoreline and mangrove thickets have been fouled by a fuel spill drifting along Puerto Rico’s southwest coast, environmental officials said Friday.

Specialists walked blackened shores as helicopters flew over the area to assess damage and track the slick, which also could threaten a pristine coral reef off the coast of La Parguera, a diving site popular with tourists.

Authorities were still investigating the origin of the spill, which stretched along an estimated eight miles of shoreline from the southwestern town of Guanica to Guayanilla Bay.

“The impact area of this spill has been quite extensive,” said Javier Velez Arocho, secretary of the Department of Natural and Environmental Resources, which has closed beaches in the area.

So far, U.S. Fish & Wildlife Service personnel had not received any reports of birds or fish being killed by the spill.