NEW ORLEANS (AP) -- Officials say underwater inspections at the site of BP's Deepwater Horizon rig disaster have failed to identify the source of a persistent sheen on the surface of the Gulf of Mexico.

The Coast Guard said Tuesday that the recent inspections confirmed BP's Macondo well, which blew out in April 2010 and spawned the nation's worst offshore oil spill, remains secure and isn't leaking any oil.

However, investigators collected samples of a white, cloudy substance that appeared to be coming from several areas on the overturned rig on the sea floor. The substance is not believed to be oil.

Remote-operated vehicles were used to inspect the rig, a riser that once connected the rig to the sea floor and a steel container lowered over a leaking drill pipe after the spill.