Close to a year after the explosion aboard the Deepwater Horizon rig flooded the Gulf of Mexico with oil, officials are responding to another — but much smaller — spill off the Louisiana coast.

"This is not a large incident in terms of the amount of product, but it is spread out over a large area," said Coast Guard Capt. Jonathan Burton at a press conference on Tuesday. "We have about a quarter to a half-mile, if you put it together, that has product spread out over a 30-mile line along the coast there." 

Reports of an oil-like substance washing up on shore in the area of Grand Isle began to come in late Saturday and early Sunday, he said. The sheen on the surface of the water extends out for up to 5 miles (8 kilometers), he said.

As of Wednesday evening, it was not yet clear how much oil had been released into the Gulf, but the spill has not had any impact on maritime traffic and no reports of harm to wildlife have emerged, said Petty Officer Steve Lehmann of the Coast Guard's Eighth District.

Testing revealed that the substance in the water was indeed Louisiana crude, eliminating the possibility that a ship had discharged a refined product, Lehmann said. The sample had a signature that made it a very close match for crude oil produced by a well owned by Anglo-Suisse Offshore Partners, a Houston-based oil and gas company.

While Anglo-Suisse has agreed to assist with cleanup efforts, the company denied they were the source of the oil, and the Coast Guard is not yet ready to name it the responsible party, Lehmann told LiveScience.

"We don't do that unless we have 100 percent certainty and we are not at that point yet," he said.

The Grand Isle coast is still being monitored after being affected by the Deepwater Horizon disaster almost a year ago, when an explosion aboard a drilling rig killed 11 people and released roughly 5 million barrels of oil into the Gulf of Mexico. People involved in this monitoring helped report the newest spill, Burton said.

The Coast Guard is also investigating discoloration on the water Chandeleur Sound, located on the opposite side of the Mississippi River from Grand Isle. It is not yet clear if that discoloration has been caused by an oil spill, he said.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry.