Oil surrounds a surfacing Portuguese man-of-war in the waters near South Pass, La. The Deepwater Horizon spill has taken an emotional toll on many people, with some describing the damage in the Gulf of Mexico as a "sacred loss" of fragile environments and endangered species. (Carol Guzy/THE WASHINGTON POST)

BP said Thursday that an oil sheen detected in the Gulf of Mexico last month probably came from crude trapped in the giant coffer dam that the company used in a futile attempt to capture the oil that was spilling from a BP well in 2010.

The Coast Guard ordered BP and drilling rig operator TransOcean to send a remotely operated vehicle to inspect the well site, which lies one mile beneath the water’s surface and 50 miles off the coast of Louisiana. BP said the inspection showed that oil was not leaking from the capped Macondo well, which blew out April 20, 2010, killing 11 men and causing the largest oil spill in U.S. history.

BP pointed to the coffer dam — an 86-ton, 40-foot-tall steel containment dome — that was lowered over a leaking pipe at the Macondo well site in May 2010 in an attempt to funnel the oil to the surface. The effort failed when a mixture of oil and slushy methane hydrates lifted the contraption off the bottom, rendering it useless.

BP said Thursday that some of the mixture remained trapped inside the coffer dam, which has been lying on the seafloor. Video taken over three days by the remotely operated vehicle showed “small, intermittent drops of oil coming from an opening at the top and another on one side of the coffer dam,” BP said.

The company said it would compare samples of the droplets it collected from the opening at the top of the coffer dam with oil from the surface sheen first seen in September.

The Coast Guard supervised the ROV inspection.

BP said it also examined the 4,500-foot pipe known as the riser and the relief well. It said it did not detect any oil leaks there. BP said it was the third time it had examined the Macondo well since it was sealed in September 2010.