On Thursday, U.S. Rep. Ed Markey, D-Mass., demanded that BP be forced to conduct new surveys of the well and the wreckage around it. He said BP should allow the public to view underwater video of its surveys in real-time.
"One can only hope that the nightmare well has not come back to haunt the people of the Gulf," Markey said. "There is no room for error, and no room for obfuscation, when it comes to this matter."
Brett Clanton, a BP spokesman, said the company had been working closely with federal agencies to investigate the sheen. He added that there was no indication that oil had leaked from the capped well.
He said tests showed the presence of alpha-olefins, a lubricant used in drilling mud and not found in oil coming directly from a reservoir. He said the presence of alpha-olefins "strongly suggests" the oil came from the wrecked riser.
Lou Colasuonno, a spokesman for Transocean, said the company would "rely on the lab analysis as to the origin of the oil." But he suggested it was BP's responsibility to deal with whatever oil remains in the riser. Transocean and BP have argued in court over responsibility for the spill.
Christopher Reddy, a chemical oceanographer at Woods Holes Oceanographic Institution, said the Coast Guard's analysis was reliable but he stressed getting the companies to investigate the leak further was paramount.
"Is it a leaky faucet that we'll have to deal with or things could get worse?" he said. "It is worth discovering."