The Coast Guard on Sunday were trying to determine whether the sheen or gleaming at the top of the water, was the result of oil or an algae growth, said Lieutenant Ryan Baxter, command duty officer in New Orleans.

"We have an unknown substance in the water," he said. "We're trying to confirm what it is."

The Coast Guard first received a report of a three-mile long rainbow sheen in the Gulf on Saturday morning. Two more reports followed, including one of a sheen that extended from six miles off the coast of Grand Isle, Louisiana, to 100 miles into the Gulf.

Coast Guard helicopters are flying over the area for an aerial survey, and the Gulf Strike Team is conducting tests, Baxter said.