The U.S. Coast Guard is investigating reports of a potentially massive oil sheen about 20 miles north of the site of last April's Deepwater Horizon oil rig explosion.

A helicopter crew and pollution investigators have been dispatched to Main Pass Block 41 in response to two calls to the National Response Center, the federal point of contact for reporting oil and chemical spills, said Paul Barnard, an operations controller for Coast Guard Sector New Orleans.

The first caller, around 11 a.m., described a sheen of about a half-mile long and a half-mile wide, he said.

About two hours later, another caller reported a much larger sheen -- about 100 miles long -- originating in the same area and spreading west to Cocodrie on Terrebonne Bay, Barnard said.

"We haven't been able to verify that, and it would be very unlikely for an individual to be able to observe a 100-mile long sheen," he said, adding inspection teams were en route around 3 p.m. to the site.

Eileen Angelico of the federal Bureau of Ocean Energy, Management, Regulation and Enforcement, which oversees offshore oil and natural gas production, said late Saturday afternoon that her agency was awaiting Coast Guard confirmation of the nature of the sheen. The bureau had not received word from any operators in the gulf of a spill, she said.