HOUSTON -- Crude oil is not leaking from the site of the 2010 Gulf of Mexico oil spill and therefore is not the cause of a mysterious sheen seen on the water surface there, according to results of an investigation led by the U.S. Coast Guard.

But investigators did discover another strange substance leaking from several parts of the wreckage from the Deepwater Horizon disaster. Samples of the substance have been collected and will be analyzed in a laboratory, officials said.

The oil sheen first spotted by BP officials in September remains in the Gulf, but the Coast Guard says it still doesn't know what is causing it. The Coast Guard says it will continue to track the sheen as it moves through the Gulf's waters while it considers other response options.

So far, the Coast Guard says that the sheen shows no signs that it will come into contact with any coastal environments and that cleaning up the sheen isn't feasible for the time being.

Oil sheens occur naturally in the Gulf, but the size and proximity of this sheen to the Macondo well site have prompted a series of subsea investigations using remotely operated vehicles (ROVs) since October.

An earlier oil leak that was discovered coming from a containment dome used in the 2010 spill response was recently sealed. But the sheen on the surface persists.

So last week BP, Transocean and the Coast Guard launched another four-day investigation using ROVs. Officials announced the end of the latest effort yesterday, and again turned up no signs of what is causing the floating sheen.

"No apparent source of the surface sheen has been discovered by this effort," federal on-scene coordinator Capt. Duke Walker said in a statement.

BP also confirmed that the Macondo wellhead itself is secure and not leaking any oil. Adjacent relief wells are also capped and secure and are not the source of any leaks, the company said.

Nevertheless, the company reported discovering "a white, cloudy substance appeared to be emanating from several places on the overturned rig." BP said it will test the substance to determine what it is and where it is coming from.

"BP will review the results of the investigation with the Coast Guard," the company said.