A sheen on the Gulf of Mexico likely came from oil seeping out of a piece of discarded equipment that failed to contain BP PLC’s massive 2010 oil spill, the company and the Coast Guard said Thursday.

A statement from BP said a three-day inspection confirmed that its Macondo well, which blew out and led to the nation’s worst offshore oil spill, isn’t leaking. A relief well that intercepted the blown-out well and sealed it isn’t leaking either, BP said.

The company said the survey determined oil probably leaked from an 86-ton steel container the company lowered over a leaking drill pipe in efforts to funnel oil to the surface. The container was one of several methods BP used in its months-long effort to contain its blown-out well.

A sheen appeared in September on the Gulf and was linked by laboratory tests to the Macondo well.

The Coast Guard said oil samples collected on Wednesday will be analyzed to determine if the container is the source of the sheen. “The Coast Guard is further evaluating what is believed to be seepage from the containment dome to determine how best to respond,” Capt. Duke Walker said in a statement.

BP said this is the third time since the well was sealed in September 2010 that it has been inspected and confirmed not to be leaking.

The Coast Guard said underwater video showed apparent oil globules leaking from the steel container at an estimated rate of less than 100 gallons per day. The container is about 1,600 feet from the wellhead.

“It is entirely separate from the wellhead and any riser piping,” the Coast Guard said. “Out of an abundance of caution, (a remotely operated vehicle) also inspected the original Macondo well area including the wreckage, debris, relief wells, and the riser on the sea floor and observed no oil leakage from that area.”

The survey was conducted jointly by BP and Transocean Ltd, owner of the rig that sank after the blown-out well triggered an explosion that killed 11 workers. The Coast Guard oversaw the inspection.