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)

The oil in a slick detected in the Gulf of Mexico last month matched oil from the Deepwater Horizon spill two years ago, the Coast Guard said Wednesday night, ending one mystery and creating another.

“The exact source of the oil is unclear at this time but could be residual oil associated with the wreckage or debris left on the seabed from the Deepwater Horizon incident,” the Coast Guard said.

The Coast Guard added that “the sheen is not feasible to recover and does not pose a risk to the shoreline.” One government expert said the thin sheen, just microns thick, was 3 miles by 300 yards on Wednesday.

Some oil drilling experts said it was unlikely that BP’s Macondo well, which suffered a blowout on April 20, 2010, was leaking again given the extra precautions taken when it was finally sealed after spilling nearly 5 million barrels of crude into the gulf.

BP declined to comment. But a BP internal slide presentation said the new oil sheen probably came from the riser, a long piece of pipe that had connected the drilling rig to the well a mile below the sea surface.

The presentation said that “the size and persistence of this slick, the persistent location of the oil slick origin point, the chemistry of the samples taken from the slick ... suggest that the likely source of the slick is a leak of Macondo ... oil mixed with drilling mud that had been trapped in the riser of the Deepwater Horizon rig.”

But Ian MacDonald, a professor of oceanography at Florida State University and a spill expert, cautioned said that the origin of the new oil remains uncertain. “The jury is out here,” he said, adding that it was too early “to rule out that this is oil freshly released from the reservoir.”

The sheen, located about 50 miles off Louisiana’s shore in the Mississippi Canyon block 252 where the Macondo well was drilled, was detected in satellite images taken on Sept. 9 and Sept. 14. The Coast Guard said the size of the sheen has varied with weather conditions.

Samples of the crude were collected and sent to a Coast Guard laboratory in New London, Conn. On Tuesday, the Coast Guard told BP and Transocean, owner and operator of the Deepwater Horizon drilling rig that caught fire and sank, that the oil from the sheen and spill matched.

In a meeting Wednesday, the Coast Guard told the companies to come up with a plan of action for determining the source. “No one’s 100 percent as to where it’s coming from,” said Frank Csulak, scientific support coordinator for the National Oceanic and Atmospheric Administration.

Since the disaster in 2010, which killed 11 workers, the wreckage of the massive rig, the crumpled riser and some hardware used in the attempt to kill the well have remained on the gulf floor. T

An August 2011 investigation, which came after oil blobs were observed on the surface and which included a visit to the wellhead by a remotely operated vehicle, turned up no sign that the well was leaking. That inspection was conducted by BP with federal government officials observing the process.

Nonetheless, there have been persistent rumors and allegations on blogs that Macondo is not truly dead, and that it is continuing to spew oil into the gulf.

Marcia McNutt, director of the U.S. Geological Survey, said a rough calculation showed that the riser, if full of oil, could hold about 1,000 barrels of oil. Because it’s open on two ends it is unlikely to have that much oil, she said.

McNutt said it’s unlikely that oil came from the deep reservoir. The well was plugged from both the top and the bottom, and has a mile of cement crammed into it.

“With what we did to it, it’s pretty hard to imagine, ” McNutt said.