The U.S. Coast Guard and BP are yet to solve the mystery behind a sheen on the surface of the Gulf of Mexico, near the Deepwater Horizon spill site.

The deepwater horizon spill, also called the BP oil spill, was one of the major environmental disasters in the history of U.S. that killed at least 11 people in 2010.

Millions of gallons of oil leaked in the Gulf, following an explosion of BP's Macondo well. Ever since the oil well was sealed permanently, regular inspections at the oil spill site are carried out to check if the Macondo well is secure and not leaking.

Experts used robotic submarines to keep a tab on the oil rig. During the latest four-day survey, researchers noticed that the well is secure and is not leaking oil, reports Associated Press.

The experts are not able to identify the source behind the Gulf sheen on the surface of the water, which was first spotted in September in block 252 of the Mississippi Canyon, about 50 miles (80 km) off the Louisiana coast, according to Reuters.

The Gulf sheen was initially thought to be caused by an 86-ton steel container that was placed on a leaking drill pipe after the disaster.

Despite capping the container, BP reported occurrence of a mile-long sheen in November again, during an overflight inspection of the oil spill site. 

During the recent survey, experts collected a white, cloudy substance that appeared to come from several places off the overturned oil rig. The samples have been sent for laboratory tests, cited the Associated Press report.

"No apparent source of the surface sheen has been discovered by this effort," Coast Guard Capt. Duke Walker said in a statement.

"Next steps are being considered as we await the lab results of the surface and subsurface samples and more detailed analysis of the video shot during the mission," he said.