“We have never seen anything like this at any of the deep coral sites that we’ve been to,” Dr. Fisher said. “And we’ve been to quite a lot of them.”

Further study is needed to conclusively link the coral die-off to the spill, scientists said, and the survey team took a number of samples from the site to test for the presence of hydrocarbons and dispersant.

Whether these samples will yield direct evidence leading back to the spill is unclear. “No one yet knows if the signature of whatever toxin killed these corals can be found in their skeletons after the tissue sloughs off,” Dr. Fisher said.

Jane Lubchenco, the administrator of the National Oceanic and Atmospheric Administration, called the findings significant. “Given the toxic nature of oil and the unprecedented amount of oil spilled, it would be surprising if we did not find damage,” she said in a statement. “This is precisely why we continue to actively monitor and evaluate the impact of the spill in the gulf.”

“We are determined to hold the responsible parties accountable for the damage done to the environment,” she added.

The ocean floor near the site of the well is still largely unexplored and is probably home to many other deep-water coral communities that scientists are eager to study.

The scientists will return to the same region on an expedition in December for more research, using a Navy vehicle that can accommodate two scientists and a pilot to depths of up to nearly 15,000 feet. Work on deep-water corals is typically conducted using advanced submersibles or remotely operated underwater vehicles.