The 2010 BP oil spill, the largest oil spill in U.S. history, is tied to an unusually high number of bottlenose dolphins washing up dead on beaches in the northern Gulf of Mexico, according to a new report by federal researchers.

Dolphins that beached in Louisiana, Mississippi and Alabama had injuries consistent with breathing and metabolizing oil, said the report released Wednesday by the National Oceanic and Atmospheric Administration. It said that most of those dolphins had lesions in their lungs and shrunken adrenal glands, which help regulate body function. The three states bore the brunt of the 130 million-gallon oil spill from BP’s Macondo well.

Forty-six bottlenose dolphins from the northern gulf states underwent necropsies for the study. The results were compared with a similar evaluation of 106 bottlenose dolphins that beached outside the northern gulf, mostly on Florida's west coast. Dolphins in the north were 22 percent more likely to suffer from bacterial pneumonia and 33 percent more likely to suffer from thin adrenal glands.

“In 70 percent of dolphins with primary bacterial pneumonia, the condition either caused or contributed significantly to death,” according to the report published in the peer-reviewed online journal PLOS One. “The rare, life-threatening, and chronic adrenal gland and lung diseases identified in stranded dolphins are consistent with exposure to petroleum compounds as seen in other mammals.”

The prevalence of injuries and disease in the group exposed to oil “was significantly higher” than the group with far less exposure, said Kathleen M. Colegrove, one of the report’s co-authors. “The thinning is very unusual,” she said. Cancer, tuberculosis and auto-immune diseases can have a similar impact on adrenal glands, “but none of it was found in dolphins in the study. More than one in five dolphins had bacterial pneumonia, most were severe and contributed to death.

“These had some of the most severe lung disease I have ever seen in the U.S.,” said Colegrove, an associate professor at the Veterinary Diagnostic Laboratory of the University of Illinois.

Most of the dead dolphins were found beached in the area most affected by the spill: Barataria Bay in Louisiana. “The timing, location, and nature of the detected lesions support that contaminants from the Deepwater Horizon oil spill caused these lesions and contributed to the high numbers of dolphin deaths within this oil spill’s footprint,” NOAA said in a statement about the report. “Increased dolphin deaths following the oil spill are part of the northern Gulf of Mexico unusual mortality event investigation.”

A spokesman for BP, Geoff Morrell, took issue with the finding. “The data we have seen thus far, including the new study from NOAA, do not show that oil from the Deepwater Horizon accident caused an increase in dolphin mortality,” Morrell said.

BP says the report failed to definitively link “the illnesses in some dolphins” to exposure to oil that poured from its well, Morrell said. In fact, he said, other studies over several decades before the oil spill have shown that respiratory illness is a common cause of death in bottlenose dolphins. “Based on a review of available literature, we are unaware of any toxicological studies linking lung disease in bottlenose dolphins to exposure to oil or other environmental contaminants,” he said.

The oil giant faces billions of dollars in legal claims and fines if science establishes a link between its oil and the deaths of marine mammals. Federal and state officials are still working with BP to determine the extent of damage to natural resources resulting from the spill and what projects are needed to restore the area, NOAA said.

An explosion rocked BP’s Deepwater Horizon oil well off Louisiana five years ago, killing 11 workers and sending petroleum gushing from a well head 5,000 feet below the surface. BP has paid billions of dollars for claims related to the spill.

NOAA and scientists who study the gulf have said an average of about 65 dolphins died per year between 2002 and 2009. In 2010 the toll nearly doubled to 125 and shot to 335 the following year.

Wednesday’s report is one in a series of federal studies related to the Deepwater Horizon disaster.

A report in 2013 found that bottlenose dolphins in Louisiana’s oil-soaked Barataria Bay have lung damage and adrenal hormone abnormalities not previously seen in other dolphin populations. That study was published in the journal Environmental Science and Technology.

“Dolphins that breathe at the water’s surface are especially vulnerable because they take deep breaths, said Stephanie Venn-Watson, a researcher at the National Marine Mammal Foundation who led the latest report.

“The dolphin habitat was known to be impacted” and there was a “good chance of inhaling oil,” she said. “Increased death occurred in areas where the spill occurred. No feasible alternative causes remain that can explain the timing, nature and increase in death.”