One of the biggest mysteries following the Deepwater Horizon oil blowout in April 2010 has been solved. Scientists are now confident the abnormally high numbers of bottlenose dolphins (Tursiops truncatus) that have died—and continue to die—in the oil spill area are suffering from ailments caused by oil byproducts. Although researchers don’t have exact numbers of how many dolphins perished following the spill, they’ve found 1281 stranded and dead between 30 April 2010 and 17 May 2015—the highest number ever recorded in the Gulf of Mexico. Barataria Bay, Louisiana, was particularly hard hit by the spill. Half of the dead dolphins found in this region between June 2010 and November 2012 had a thin adrenal gland cortex—a key indicator of an ailment known as adrenal insufficiency which often leads to death in dolphins, particularly among those who are pregnant. This same lesion was found in one of every three dolphins examined in oil-contaminated areas across Louisiana, Mississippi, and Alabama, the team reports today in PLOS ONE. In comparison, only 7% of 106 dolphins found stranded in other coastal areas away from the spill had this adrenal gland ailment. In addition, the researchers found that more than one in five dolphins that died in the Deepwater Horizon spill area and time frame had bacterial pneumonia. The lead pathologist of the study says that the lung lesions were some of the most severe she has examined in 13 years of analyzing dolphin lung tissue from animals in U.S. waters. Oil products are known to injure the lungs and alter the immune function of mammals, leading to an increased susceptibility to pneumonia. And dolphins are particularly susceptible because of their large lungs, deep breathing, and need to hold their breath for long periods.