CAIN BURDEAUNEW ORLEANS (AP) â¿¿ Federal scientists and BP say oil appears to have leaked last month from the drilling wreckage lying at the bottom of the Gulf of Mexico near where a BP well blew out in 2010, causing the nation's worst offshore spill. A probe started after a sheen was discovered Sept. 16 in the waters near the site indicates the oil may have seeped from a mile-long metal tube, called the riser, which connected the Deepwater Horizon drilling rig to the Macondo well. The well itself, capped after more than 200 million gallons of oil spewed, is not believed to be leaking, officials said. The oil sheen posed no environmental threat because it was a small amount of oil and was far from land, federal officials said. The announcement of oil leaking out comes at a sensitive time as BP and the Justice Department negotiate terms of a possible settlement to resolve government's claims against the oil giant. Several billion dollars are at stake if the talks produce a settlement for what likely will be record-setting civil and criminal penalties. The tests of the sheen showed chemical signatures of Macondo oil mixed with drilling muds, which are lubricants used both in drilling the well and trying to plug it after the April 20, 2010 explosion. "This led to the conclusion that the oil causing the sheen out at the surface is likely coming from the riser. The riser has drilling muds in it," said Frank Csulak, the scientific support coordinator for the National Oceanic and Atmospheric Administration. He said at most the riser could contain about 75,600 gallons of oil, but experts expect less than that is trapped inside it. He said tests have ruled out the possibility that the new sheen came from oil sitting on the bottom of the Gulf because it was not weathered and degraded.