NEW ORLEANS (AP) — BP PLC says a sheen on the Gulf of Mexico appears to have come from oil seeping out of a piece of discarded equipment used to contain its massive 2010 oil spill.

On Thursday, the company said underwater surveys determined oil probably leaked from an 86-ton steel cofferdam the company lowered down to the well in efforts to funnel oil to the surface. The cofferdam was one of several methods BP used in its months-long effort to contain its blown-out Macondo well.

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

BP says that in the past three days, it sent remotely operated vehicles down to the wreck site and found that its blown-out well was secure.