Despite underwater inspections at the site of the 2010 Deepwater Horizon explosion, BP has failed to identify the source of a persistent oil sheen in the Gulf of Mexico, officials announced Tuesday.

Recent inspections confirmed that the Macondo well is secure, both BP and the U.S. Coast Guard said. The well, which blew out in 2010 and poured millions of gallons of oil into the Gulf in the nation’s worst offshore oil spill, is not leaking oil, and the relief wells that were drilled to stop the gush also are still secure, BP said after the four-day subsea survey.

The rig, wreckage and containment dome were all inspected by robot submarines, with a focus on determining whether oil from the 2010 spill was still trapped within the wreckage. The inspection was the fourth since the well was permanently sealed in September 2010.

“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.”