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."