HOUSTON -- Another inspection of the scene of the 2010 Deepwater Horizon drilling rig fire and oil leak is under way in the Gulf of Mexico to determine what's causing an oil slick found on the surface.

BP PLC and Transocean Ltd. are once again employing remote-operated vehicles (ROVs) to inspect the site and the wreckage lying a mile beneath the surface on the ocean floor where the Macondo well spilled nearly 5 million barrels of crude oil into the Gulf two years ago. The Coast Guard is monitoring the work.

The ROV investigation starting today is the latest in a series of surveys taken by the companies to monitor the site.

An investigation in October determined that an oil sheen near the site of the spill was caused by crude oil leaking from the old containment dome that's resting at the ocean bottom. BP first used the containment dome to try to stanch the uncontrolled flow of oil from Macondo.

BP crews plugged that leak, but Coast Guard officials say an oil sheen remains visible at the site. BP and Transocean developed a new plan to jointly determine the source of the sheen and ways to mitigate the pollution, which was submitted Nov. 9 for the Coast Guard to review.

The Coast Guard reported that it approved the plan Thursday.

"The plan calls for ROVs to inspect potential sources of oil in the vicinity of the Deepwater Horizon wellhead and rig wreckage to address the sheen that persists in the area," Coast Guard officials said in a statement. "In addition, Captain Duke Walker, Federal On-Scene Coordinator (FOSC) for the Deepwater Horizon oil spill response, tasked BP and Transocean with developing a comprehensive array of options for permanent remediation of oil that could still be contained within wreckage in the vicinity."

BP says the investigation starting today will have ROVs thoroughly inspecting the collapsed Deepwater Horizon rig and all other wrecked equipment located at the site, included crumpled riser piping. Representatives there declined to speculate on what may be causing the sheen that the Coast Guard grew concerned about in early November.

"A further investigation is planned to inspect the Deepwater Horizon rig and associated wreckage," BP spokesman Brett Clanton said in an email. "If it is identified as a potential source of sheen, we will work with the Coast Guard and rig owner Transocean to address the matter."

Last week, U.S. EPA announced that it was banning BP from participating in any new federal contracting, citing the company's "lack of business integrity." The move also prevents BP from bidding for acreage in new Gulf of Mexico leasing rounds (Greenwire, Nov. 28).

BP assured investors that the move does not affect any current BP government contracts or existing operations.