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, officials said Tuesday.

The Coast Guard and BP both said the recent inspections confirmed that the company's Macondo well, which blew out in April 2010 and spawned the nation's worst offshore oil spill, remains secure and isn't leaking oil. Relief wells that were drilled in 2010 to stop the gusher also were found to be secure during the four-day survey, BP said.

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. Lab tests were planned on the samples of the substance, which isn't believed to be oil.

"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."

Robot submarines were used to inspect the rig, portions of the riser that once connected the rig to the sea floor and an 86-ton steel container that was lowered over a leaking drill pipe in the spill's aftermath. The survey was focused on seeing if any oil from the 2010 spill is still trapped in the wreckage.

BP said the latest survey, which ended Saturday, marked the fourth time since the well was permanently sealed in September 2010 that inspections have confirmed it isn't leaking.

After a sheen was spotted near the site of the blowout in September, workers capped and plugged the steel container that was suspected to be the source. On Nov. 2, however, BP reported another sheen in the same area.

The Coast Guard has said the sheen can't be recovered and doesn't pose a risk to the shoreline.

The Macondo well was capped on July 15, 2010, and permanently sealed with cement on Sept. 19, 2010.