HOUSTON -- The U.S. Coast Guard said late Saturday that it is investigating reports of a miles-long oil slick in the Gulf of Mexico.

The Coast Guard said in a news release that it received a report of a three-mile-long rainbow sheen off the Louisiana coast just before 9:30 a.m. local time on Saturday. Two subsequent sightings were relayed to the Coast Guard, the last of which reported a sheen that extended from about 6 miles south of Grand Isle, La. to 100 miles offshore.

Though the Coast Guard was able to confirm that there is a substance on the water's surface, it has not yet been able to determine if it is oil. Petty Officer Casey Ranel said that those that observed the sheen from a helicopter said they saw no sheen associated with the substance. That flight was diverted from the scene on a separate search and rescue mission, however, and could not continue their investigation, the Coast Guard said in the news release.

The Coast Guard has since launched additional aircraft and boats to the scene from New Orleans and Mobile, Ala., to collect samples of the substance. Ranel said the area where the substance has been reported is about 20 miles west of where the Deepwater Horizon rig exploded last April, killing 11 and unleashing the largest offshore oil spill in U.S. history.

Ranel said the Coast Guard has not linked the substance to any particular rig or well.